diff --git a/DESCRIPTION b/DESCRIPTION
index 8d2562e1c08..381fb1ecae1 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -30,6 +30,7 @@ Depends:
     methods,
     R (>= 3.0.2)
 Imports:
+    cli,
     graphics,
     grDevices,
     magrittr,
diff --git a/R/aaa-auto.R b/R/aaa-auto.R
index 9636328d32f..71cebbcfd39 100644
--- a/R/aaa-auto.R
+++ b/R/aaa-auto.R
@@ -2,7 +2,7 @@
 
 vcount_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -241,7 +241,7 @@ hsbm_list_game_impl <- function(n, mlist, rholist, Clist, p) {
 
 correlated_game_impl <- function(old.graph, corr, p=edge_density(old.graph), permutation=NULL) {
   # Argument checks
-  if (!is_igraph(old.graph)) { stop("Not a graph object") }
+  ensure_igraph(old.graph)
   corr <- as.numeric(corr)
   p <- as.numeric(p)
   if (!is.null(permutation)) permutation <- as.numeric(permutation)-1
@@ -328,7 +328,7 @@ sample_dirichlet_impl <- function(n, alpha) {
 
 harmonic_centrality_cutoff_impl <- function(graph, vids=V(graph), mode=c("out", "in", "all", "total"), weights=NULL, normalized=FALSE, cutoff=-1) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   vids <- as.igraph.vs(graph, vids)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
@@ -353,7 +353,7 @@ harmonic_centrality_cutoff_impl <- function(graph, vids=V(graph), mode=c("out",
 
 personalized_pagerank_impl <- function(graph, algo=c("prpack", "arpack"), vids=V(graph), directed=TRUE, damping=0.85, personalized=NULL, weights=NULL, options=NULL) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   algo <- switch(igraph.match.arg(algo), "arpack"=1L, "prpack"=2L)
   vids <- as.igraph.vs(graph, vids)
   directed <- as.logical(directed)
@@ -388,7 +388,7 @@ personalized_pagerank_impl <- function(graph, algo=c("prpack", "arpack"), vids=V
 
 reverse_edges_impl <- function(graph, eids=E(graph)) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   eids <- as.igraph.es(graph, eids)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -400,7 +400,7 @@ reverse_edges_impl <- function(graph, eids=E(graph)) {
 
 average_path_length_dijkstra_impl <- function(graph, weights=NULL, directed=TRUE, unconnected=TRUE, details=FALSE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
@@ -423,7 +423,7 @@ average_path_length_dijkstra_impl <- function(graph, weights=NULL, directed=TRUE
 
 path_length_hist_impl <- function(graph, directed=TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   directed <- as.logical(directed)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -435,7 +435,7 @@ path_length_hist_impl <- function(graph, directed=TRUE) {
 
 simplify_impl <- function(graph, remove.multiple=TRUE, remove.loops=TRUE, edge.attr.comb=igraph_opt("edge.attr.comb")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   remove.multiple <- as.logical(remove.multiple)
   remove.loops <- as.logical(remove.loops)
   edge.attr.comb <- igraph.i.attribute.combination(edge.attr.comb)
@@ -449,7 +449,7 @@ simplify_impl <- function(graph, remove.multiple=TRUE, remove.loops=TRUE, edge.a
 
 feedback_arc_set_impl <- function(graph, weights=NULL, algo=c("approx_eades", "exact_ip")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
@@ -471,7 +471,7 @@ feedback_arc_set_impl <- function(graph, weights=NULL, algo=c("approx_eades", "e
 
 is_loop_impl <- function(graph, eids=E(graph)) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   eids <- as.igraph.es(graph, eids)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -483,7 +483,7 @@ is_loop_impl <- function(graph, eids=E(graph)) {
 
 is_dag_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -494,7 +494,7 @@ is_dag_impl <- function(graph) {
 
 is_simple_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -505,7 +505,7 @@ is_simple_impl <- function(graph) {
 
 is_multiple_impl <- function(graph, eids=E(graph)) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   eids <- as.igraph.es(graph, eids)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -517,7 +517,7 @@ is_multiple_impl <- function(graph, eids=E(graph)) {
 
 has_loop_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -528,7 +528,7 @@ has_loop_impl <- function(graph) {
 
 has_multiple_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -539,7 +539,7 @@ has_multiple_impl <- function(graph) {
 
 count_multiple_impl <- function(graph, eids=E(graph)) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   eids <- as.igraph.es(graph, eids)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -551,7 +551,7 @@ count_multiple_impl <- function(graph, eids=E(graph)) {
 
 eigenvector_centrality_impl <- function(graph, directed=FALSE, scale=TRUE, weights=NULL, options=arpack_defaults) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   directed <- as.logical(directed)
   scale <- as.logical(scale)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
@@ -575,7 +575,7 @@ eigenvector_centrality_impl <- function(graph, directed=FALSE, scale=TRUE, weigh
 
 hub_score_impl <- function(graph, scale=TRUE, weights=NULL, options=arpack_defaults) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   scale <- as.logical(scale)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -598,7 +598,7 @@ hub_score_impl <- function(graph, scale=TRUE, weights=NULL, options=arpack_defau
 
 authority_score_impl <- function(graph, scale=TRUE, weights=NULL, options=arpack_defaults) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   scale <- as.logical(scale)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -621,7 +621,7 @@ authority_score_impl <- function(graph, scale=TRUE, weights=NULL, options=arpack
 
 is_mutual_impl <- function(graph, eids=E(graph)) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   eids <- as.igraph.es(graph, eids)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -633,7 +633,7 @@ is_mutual_impl <- function(graph, eids=E(graph)) {
 
 maximum_cardinality_search_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -646,7 +646,7 @@ maximum_cardinality_search_impl <- function(graph) {
 
 avg_nearest_neighbor_degree_impl <- function(graph, vids=V(graph), mode=c("all", "out", "in", "total"), neighbor.degree.mode=c("all", "out", "in", "total"), weights=NULL) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   vids <- as.igraph.vs(graph, vids)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
   neighbor.degree.mode <- switch(igraph.match.arg(neighbor.degree.mode), "out"=1, "in"=2, "all"=3, "total"=3)
@@ -670,7 +670,7 @@ avg_nearest_neighbor_degree_impl <- function(graph, vids=V(graph), mode=c("all",
 
 strength_impl <- function(graph, vids=V(graph), mode=c("all", "out", "in", "total"), loops=TRUE, weights=NULL) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   vids <- as.igraph.vs(graph, vids)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
   loops <- as.logical(loops)
@@ -708,7 +708,7 @@ centralization_impl <- function(scores, theoretical.max=0, normalized=TRUE) {
 
 centralization_degree_impl <- function(graph, mode=c("all", "out", "in", "total"), loops=TRUE, normalized=TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
   loops <- as.logical(loops)
   normalized <- as.logical(normalized)
@@ -722,7 +722,7 @@ centralization_degree_impl <- function(graph, mode=c("all", "out", "in", "total"
 
 centralization_betweenness_impl <- function(graph, directed=TRUE, normalized=TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   directed <- as.logical(directed)
   normalized <- as.logical(normalized)
 
@@ -735,7 +735,7 @@ centralization_betweenness_impl <- function(graph, directed=TRUE, normalized=TRU
 
 centralization_betweenness_tmax_impl <- function(graph=NULL, nodes=0, directed=TRUE) {
   # Argument checks
-  if (!is.null(graph) && !is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph, optional = TRUE)
   nodes <- as.integer(nodes)
   directed <- as.logical(directed)
 
@@ -748,7 +748,7 @@ centralization_betweenness_tmax_impl <- function(graph=NULL, nodes=0, directed=T
 
 centralization_closeness_impl <- function(graph, mode=c("out", "in", "all", "total"), normalized=TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
   normalized <- as.logical(normalized)
 
@@ -761,7 +761,7 @@ centralization_closeness_impl <- function(graph, mode=c("out", "in", "all", "tot
 
 centralization_closeness_tmax_impl <- function(graph=NULL, nodes=0, mode=c("out", "in", "all", "total")) {
   # Argument checks
-  if (!is.null(graph) && !is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph, optional = TRUE)
   nodes <- as.integer(nodes)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
 
@@ -774,7 +774,7 @@ centralization_closeness_tmax_impl <- function(graph=NULL, nodes=0, mode=c("out"
 
 centralization_eigenvector_centrality_impl <- function(graph, directed=FALSE, scale=TRUE, options=arpack_defaults, normalized=TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   directed <- as.logical(directed)
   scale <- as.logical(scale)
   options.tmp <- arpack_defaults; options.tmp[ names(options) ] <- options ; options <- options.tmp
@@ -789,7 +789,7 @@ centralization_eigenvector_centrality_impl <- function(graph, directed=FALSE, sc
 
 centralization_eigenvector_centrality_tmax_impl <- function(graph=NULL, nodes=0, directed=FALSE, scale=TRUE) {
   # Argument checks
-  if (!is.null(graph) && !is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph, optional = TRUE)
   nodes <- as.integer(nodes)
   directed <- as.logical(directed)
   scale <- as.logical(scale)
@@ -803,7 +803,7 @@ centralization_eigenvector_centrality_tmax_impl <- function(graph=NULL, nodes=0,
 
 assortativity_nominal_impl <- function(graph, types, directed=TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   types <- as.numeric(types)-1
   directed <- as.logical(directed)
 
@@ -816,7 +816,7 @@ assortativity_nominal_impl <- function(graph, types, directed=TRUE) {
 
 assortativity_impl <- function(graph, types1, types2=NULL, directed=TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   types1 <- as.numeric(types1)
   if (!is.null(types2)) types2 <- as.numeric(types2)
   directed <- as.logical(directed)
@@ -830,7 +830,7 @@ assortativity_impl <- function(graph, types1, types2=NULL, directed=TRUE) {
 
 assortativity_degree_impl <- function(graph, directed=TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   directed <- as.logical(directed)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -842,7 +842,7 @@ assortativity_degree_impl <- function(graph, directed=TRUE) {
 
 contract_vertices_impl <- function(graph, mapping, vertex.attr.comb=igraph_opt("vertex.attr.comb")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   mapping <- as.numeric(mapping)-1
   vertex.attr.comb <- igraph.i.attribute.combination(vertex.attr.comb)
 
@@ -855,7 +855,7 @@ contract_vertices_impl <- function(graph, mapping, vertex.attr.comb=igraph_opt("
 
 eccentricity_impl <- function(graph, vids=V(graph), mode=c("all", "out", "in", "total")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   vids <- as.igraph.vs(graph, vids)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
 
@@ -870,7 +870,7 @@ eccentricity_impl <- function(graph, vids=V(graph), mode=c("all", "out", "in", "
 
 radius_impl <- function(graph, mode=c("all", "out", "in", "total")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -882,7 +882,7 @@ radius_impl <- function(graph, mode=c("all", "out", "in", "total")) {
 
 diversity_impl <- function(graph, weights=NULL, vids=V(graph)) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
@@ -904,7 +904,7 @@ diversity_impl <- function(graph, weights=NULL, vids=V(graph)) {
 
 random_walk_impl <- function(graph, start, steps, mode=c("out", "in", "all", "total"), stuck=c("return", "error")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   start <- as.igraph.vs(graph, start)
   if (length(start) == 0) {
     stop("No vertex was specified")
@@ -924,7 +924,7 @@ random_walk_impl <- function(graph, start, steps, mode=c("out", "in", "all", "to
 
 random_edge_walk_impl <- function(graph, start, steps, weights=NULL, mode=c("out", "in", "all", "total"), stuck=c("return", "error")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
@@ -952,7 +952,7 @@ random_edge_walk_impl <- function(graph, start, steps, weights=NULL, mode=c("out
 
 global_efficiency_impl <- function(graph, weights=NULL, directed=TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
@@ -972,7 +972,7 @@ global_efficiency_impl <- function(graph, weights=NULL, directed=TRUE) {
 
 local_efficiency_impl <- function(graph, vids=V(graph), weights=NULL, directed=TRUE, mode=c("all", "out", "in", "total")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   vids <- as.igraph.vs(graph, vids)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -996,7 +996,7 @@ local_efficiency_impl <- function(graph, vids=V(graph), weights=NULL, directed=T
 
 average_local_efficiency_impl <- function(graph, weights=NULL, directed=TRUE, mode=c("all", "out", "in", "total")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
@@ -1031,7 +1031,7 @@ is_graphical_impl <- function(out.deg, in.deg=NULL, allowed.edge.types=c("simple
 
 bipartite_projection_size_impl <- function(graph, types=NULL) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   types <- handle_vertex_type_arg(types, graph)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -1043,7 +1043,7 @@ bipartite_projection_size_impl <- function(graph, types=NULL) {
 
 is_bipartite_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1054,7 +1054,7 @@ is_bipartite_impl <- function(graph) {
 
 is_connected_impl <- function(graph, mode=c("weak", "strong")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   mode <- switch(igraph.match.arg(mode), "weak"=1, "strong"=2)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -1066,7 +1066,7 @@ is_connected_impl <- function(graph, mode=c("weak", "strong")) {
 
 articulation_points_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1079,7 +1079,7 @@ articulation_points_impl <- function(graph) {
 
 biconnected_components_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1110,7 +1110,7 @@ biconnected_components_impl <- function(graph) {
 
 bridges_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1123,7 +1123,7 @@ bridges_impl <- function(graph) {
 
 cliques_impl <- function(graph, min=0, max=0) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   min <- as.integer(min)
   max <- as.integer(max)
 
@@ -1141,7 +1141,7 @@ cliques_impl <- function(graph, min=0, max=0) {
 
 clique_size_hist_impl <- function(graph, min.size=0, max.size=0) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   min.size <- as.integer(min.size)
   max.size <- as.integer(max.size)
 
@@ -1154,7 +1154,7 @@ clique_size_hist_impl <- function(graph, min.size=0, max.size=0) {
 
 largest_cliques_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1170,7 +1170,7 @@ largest_cliques_impl <- function(graph) {
 
 maximal_cliques_hist_impl <- function(graph, min.size=0, max.size=0) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   min.size <- as.integer(min.size)
   max.size <- as.integer(max.size)
 
@@ -1183,7 +1183,7 @@ maximal_cliques_hist_impl <- function(graph, min.size=0, max.size=0) {
 
 clique_number_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1194,7 +1194,7 @@ clique_number_impl <- function(graph) {
 
 weighted_cliques_impl <- function(graph, vertex.weights=NULL, min.weight=0, max.weight=0, maximal=FALSE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (is.null(vertex.weights) && "weight" %in% vertex_attr_names(graph)) {
     vertex.weights <- V(graph)$weight
   }
@@ -1221,7 +1221,7 @@ weighted_cliques_impl <- function(graph, vertex.weights=NULL, min.weight=0, max.
 
 largest_weighted_cliques_impl <- function(graph, vertex.weights=NULL) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (is.null(vertex.weights) && "weight" %in% vertex_attr_names(graph)) {
     vertex.weights <- V(graph)$weight
   }
@@ -1245,7 +1245,7 @@ largest_weighted_cliques_impl <- function(graph, vertex.weights=NULL) {
 
 weighted_clique_number_impl <- function(graph, vertex.weights=NULL) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (is.null(vertex.weights) && "weight" %in% vertex_attr_names(graph)) {
     vertex.weights <- V(graph)$weight
   }
@@ -1264,7 +1264,7 @@ weighted_clique_number_impl <- function(graph, vertex.weights=NULL) {
 
 similarity_jaccard_impl <- function(graph, vids=V(graph), mode=c("all", "out", "in", "total"), loops=FALSE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   vids <- as.igraph.vs(graph, vids)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
   loops <- as.logical(loops)
@@ -1278,7 +1278,7 @@ similarity_jaccard_impl <- function(graph, vids=V(graph), mode=c("all", "out", "
 
 similarity_dice_impl <- function(graph, vids=V(graph), mode=c("all", "out", "in", "total"), loops=FALSE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   vids <- as.igraph.vs(graph, vids)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
   loops <- as.logical(loops)
@@ -1292,7 +1292,7 @@ similarity_dice_impl <- function(graph, vids=V(graph), mode=c("all", "out", "in"
 
 similarity_inverse_log_weighted_impl <- function(graph, vids=V(graph), mode=c("all", "out", "in", "total")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   vids <- as.igraph.vs(graph, vids)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
 
@@ -1337,7 +1337,7 @@ hrg_dendrogram_impl <- function(hrg) {
 
 hrg_consensus_impl <- function(graph, hrg=NULL, start=FALSE, num.samples=10000) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (is.null(hrg)) {
     hrg <- list(left=c(), right=c(), prob=c(), edges=c(), vertices=c())
   }
@@ -1354,7 +1354,7 @@ hrg_consensus_impl <- function(graph, hrg=NULL, start=FALSE, num.samples=10000)
 
 hrg_create_impl <- function(graph, prob) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   prob <- as.numeric(prob)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -1367,7 +1367,7 @@ hrg_create_impl <- function(graph, prob) {
 
 graphlets_impl <- function(graph, weights=NULL, niter=1000) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
@@ -1392,7 +1392,7 @@ graphlets_impl <- function(graph, weights=NULL, niter=1000) {
 
 to_directed_impl <- function(graph, mode=c("mutual", "arbitrary", "random", "acyclic")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   mode <- switch(igraph.match.arg(mode), "arbitrary"=0, "mutual"=1, "random"=2, "acyclic"=3)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -1404,7 +1404,7 @@ to_directed_impl <- function(graph, mode=c("mutual", "arbitrary", "random", "acy
 
 dyad_census_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1415,7 +1415,7 @@ dyad_census_impl <- function(graph) {
 
 triad_census_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1426,7 +1426,7 @@ triad_census_impl <- function(graph) {
 
 adjacent_triangles_impl <- function(graph, vids=V(graph)) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   vids <- as.igraph.vs(graph, vids)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -1438,7 +1438,7 @@ adjacent_triangles_impl <- function(graph, vids=V(graph)) {
 
 list_triangles_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1451,7 +1451,7 @@ list_triangles_impl <- function(graph) {
 
 maxflow_impl <- function(graph, source, target, capacity=NULL) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   source <- as.igraph.vs(graph, source)
   if (length(source) == 0) {
     stop("No vertex was specified")
@@ -1483,7 +1483,7 @@ maxflow_impl <- function(graph, source, target, capacity=NULL) {
 
 dominator_tree_impl <- function(graph, root, mode=c("out", "in", "all", "total")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   root <- as.igraph.vs(graph, root)
   if (length(root) == 0) {
     stop("No vertex was specified")
@@ -1501,7 +1501,7 @@ dominator_tree_impl <- function(graph, root, mode=c("out", "in", "all", "total")
 
 all_st_cuts_impl <- function(graph, source, target) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   source <- as.igraph.vs(graph, source)
   if (length(source) == 0) {
     stop("No vertex was specified")
@@ -1531,7 +1531,7 @@ all_st_cuts_impl <- function(graph, source, target) {
 
 all_st_mincuts_impl <- function(graph, source, target, capacity=NULL) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   source <- as.igraph.vs(graph, source)
   if (length(source) == 0) {
     stop("No vertex was specified")
@@ -1569,7 +1569,7 @@ all_st_mincuts_impl <- function(graph, source, target, capacity=NULL) {
 
 is_separator_impl <- function(graph, candidate) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   candidate <- as.igraph.vs(graph, candidate)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -1581,7 +1581,7 @@ is_separator_impl <- function(graph, candidate) {
 
 is_minimal_separator_impl <- function(graph, candidate) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   candidate <- as.igraph.vs(graph, candidate)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -1593,7 +1593,7 @@ is_minimal_separator_impl <- function(graph, candidate) {
 
 all_minimal_st_separators_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1609,7 +1609,7 @@ all_minimal_st_separators_impl <- function(graph) {
 
 minimum_size_separators_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1625,7 +1625,7 @@ minimum_size_separators_impl <- function(graph) {
 
 isoclass_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1636,8 +1636,8 @@ isoclass_impl <- function(graph) {
 
 isomorphic_impl <- function(graph1, graph2) {
   # Argument checks
-  if (!is_igraph(graph1)) { stop("Not a graph object") }
-  if (!is_igraph(graph2)) { stop("Not a graph object") }
+  ensure_igraph(graph1)
+  ensure_igraph(graph2)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1661,8 +1661,8 @@ isoclass_create_impl <- function(size, number, directed=TRUE) {
 
 isomorphic_vf2_impl <- function(graph1, graph2, vertex.color1, vertex.color2, edge.color1, edge.color2) {
   # Argument checks
-  if (!is_igraph(graph1)) { stop("Not a graph object") }
-  if (!is_igraph(graph2)) { stop("Not a graph object") }
+  ensure_igraph(graph1)
+  ensure_igraph(graph2)
   if (missing(vertex.color1)) {
     if ("color" %in% vertex_attr_names(graph1)) {
       vertex.color1 <- V(graph1)$color
@@ -1713,8 +1713,8 @@ isomorphic_vf2_impl <- function(graph1, graph2, vertex.color1, vertex.color2, ed
 
 count_isomorphisms_vf2_impl <- function(graph1, graph2, vertex.color1, vertex.color2, edge.color1, edge.color2) {
   # Argument checks
-  if (!is_igraph(graph1)) { stop("Not a graph object") }
-  if (!is_igraph(graph2)) { stop("Not a graph object") }
+  ensure_igraph(graph1)
+  ensure_igraph(graph2)
   if (missing(vertex.color1)) {
     if ("color" %in% vertex_attr_names(graph1)) {
       vertex.color1 <- V(graph1)$color
@@ -1765,8 +1765,8 @@ count_isomorphisms_vf2_impl <- function(graph1, graph2, vertex.color1, vertex.co
 
 subisomorphic_vf2_impl <- function(graph1, graph2, vertex.color1, vertex.color2, edge.color1, edge.color2) {
   # Argument checks
-  if (!is_igraph(graph1)) { stop("Not a graph object") }
-  if (!is_igraph(graph2)) { stop("Not a graph object") }
+  ensure_igraph(graph1)
+  ensure_igraph(graph2)
   if (missing(vertex.color1)) {
     if ("color" %in% vertex_attr_names(graph1)) {
       vertex.color1 <- V(graph1)$color
@@ -1817,8 +1817,8 @@ subisomorphic_vf2_impl <- function(graph1, graph2, vertex.color1, vertex.color2,
 
 count_subisomorphisms_vf2_impl <- function(graph1, graph2, vertex.color1, vertex.color2, edge.color1, edge.color2) {
   # Argument checks
-  if (!is_igraph(graph1)) { stop("Not a graph object") }
-  if (!is_igraph(graph2)) { stop("Not a graph object") }
+  ensure_igraph(graph1)
+  ensure_igraph(graph2)
   if (missing(vertex.color1)) {
     if ("color" %in% vertex_attr_names(graph1)) {
       vertex.color1 <- V(graph1)$color
@@ -1869,8 +1869,8 @@ count_subisomorphisms_vf2_impl <- function(graph1, graph2, vertex.color1, vertex
 
 isomorphic_34_impl <- function(graph1, graph2) {
   # Argument checks
-  if (!is_igraph(graph1)) { stop("Not a graph object") }
-  if (!is_igraph(graph2)) { stop("Not a graph object") }
+  ensure_igraph(graph1)
+  ensure_igraph(graph2)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -1881,7 +1881,7 @@ isomorphic_34_impl <- function(graph1, graph2) {
 
 canonical_permutation_impl <- function(graph, colors, sh=c("fm", "f", "fs", "fl", "flm", "fsm")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (missing(colors)) {
     if ("color" %in% vertex_attr_names(graph)) {
       colors <- V(graph)$color
@@ -1903,7 +1903,7 @@ canonical_permutation_impl <- function(graph, colors, sh=c("fm", "f", "fs", "fl"
 
 permute_vertices_impl <- function(graph, permutation) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   permutation <- as.numeric(permutation)-1
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -1915,8 +1915,8 @@ permute_vertices_impl <- function(graph, permutation) {
 
 isomorphic_bliss_impl <- function(graph1, graph2, colors1, colors2, sh=c("fm", "f", "fs", "fl", "flm", "fsm")) {
   # Argument checks
-  if (!is_igraph(graph1)) { stop("Not a graph object") }
-  if (!is_igraph(graph2)) { stop("Not a graph object") }
+  ensure_igraph(graph1)
+  ensure_igraph(graph2)
   if (missing(colors1)) {
     if ("color" %in% vertex_attr_names(graph1)) {
       colors1 <- V(graph1)$color
@@ -1948,7 +1948,7 @@ isomorphic_bliss_impl <- function(graph1, graph2, colors1, colors2, sh=c("fm", "
 
 automorphisms_impl <- function(graph, colors, sh=c("fm", "f", "fs", "fl", "flm", "fsm")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (missing(colors)) {
     if ("color" %in% vertex_attr_names(graph)) {
       colors <- V(graph)$color
@@ -1970,7 +1970,7 @@ automorphisms_impl <- function(graph, colors, sh=c("fm", "f", "fs", "fl", "flm",
 
 automorphism_group_impl <- function(graph, colors, sh=c("fm", "f", "fs", "fl", "flm", "fsm"), details=FALSE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   if (missing(colors)) {
     if ("color" %in% vertex_attr_names(graph)) {
       colors <- V(graph)$color
@@ -2015,7 +2015,7 @@ scg_norm_eps_impl <- function(V, groups, mtype=c("symmetric", "laplacian", "stoc
 
 adjacency_spectral_embedding_impl <- function(graph, no, weights=NULL, which=c("lm", "la", "sa"), scaled=TRUE, cvec=graph.strength(graph, weights=weights)/(vcount(graph)-1), options=igraph.arpack.default) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   no <- as.integer(no)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -2039,7 +2039,7 @@ adjacency_spectral_embedding_impl <- function(graph, no, weights=NULL, which=c("
 
 laplacian_spectral_embedding_impl <- function(graph, no, weights=NULL, which=c("lm", "la", "sa"), type=c("default", "D-A", "DAD", "I-DAD", "OAP"), scaled=TRUE, options=igraph.arpack.default) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   no <- as.integer(no)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -2066,7 +2066,7 @@ laplacian_spectral_embedding_impl <- function(graph, no, weights=NULL, which=c("
 
 eigen_adjacency_impl <- function(graph, algorithm=c("arpack", "auto", "lapack", "comp_auto", "comp_lapack", "comp_arpack"), which=list(), options=arpack_defaults) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   algorithm <- switch(igraph.match.arg(algorithm), "auto"=0, "lapack"=1,
     "arpack"=2, "comp_auto"=3, "comp_lapack"=4,
     "comp_arpack"=5)
@@ -2083,7 +2083,7 @@ eigen_adjacency_impl <- function(graph, algorithm=c("arpack", "auto", "lapack",
 
 sir_impl <- function(graph, beta, gamma, no.sim=100) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   beta <- as.numeric(beta)
   gamma <- as.numeric(gamma)
   no.sim <- as.integer(no.sim)
@@ -2120,7 +2120,7 @@ dim_select_impl <- function(sv) {
 
 is_eulerian_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -2131,7 +2131,7 @@ is_eulerian_impl <- function(graph) {
 
 eulerian_path_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -2147,7 +2147,7 @@ eulerian_path_impl <- function(graph) {
 
 eulerian_cycle_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -2163,7 +2163,7 @@ eulerian_cycle_impl <- function(graph) {
 
 is_tree_impl <- function(graph, mode=c("out", "in", "all", "total"), details=FALSE) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   mode <- switch(igraph.match.arg(mode), "out"=1, "in"=2, "all"=3, "total"=3)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -2196,7 +2196,7 @@ from_prufer_impl <- function(prufer) {
 
 to_prufer_impl <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
 
   on.exit( .Call(R_igraph_finalizer) )
   # Function call
@@ -2207,7 +2207,7 @@ to_prufer_impl <- function(graph) {
 
 random_spanning_tree_impl <- function(graph, vid=0) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   vid <- as.igraph.vs(graph, vid)
   if (length(vid) == 0) {
     stop("No vertex was specified")
@@ -2237,7 +2237,7 @@ tree_game_impl <- function(n, directed=FALSE, method=c("lerw", "prufer")) {
 
 vertex_coloring_greedy_impl <- function(graph, heuristic=c("colored_neighbors")) {
   # Argument checks
-  if (!is_igraph(graph)) { stop("Not a graph object") }
+  ensure_igraph(graph)
   heuristic <- switch(igraph.match.arg(heuristic), "colored_neighbors"=0L)
 
   on.exit( .Call(R_igraph_finalizer) )
@@ -2249,4 +2249,3 @@ vertex_coloring_greedy_impl <- function(graph, heuristic=c("colored_neighbors"))
   }
   res
 }
-
diff --git a/R/attributes.R b/R/attributes.R
index 8943ecd5092..71274306aeb 100644
--- a/R/attributes.R
+++ b/R/attributes.R
@@ -51,9 +51,7 @@
 #' graph_attr(g)
 #' graph_attr(g, "name")
 graph_attr <- function(graph, name) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (missing(name)) {
     graph.attributes(graph)
   } else {
@@ -112,26 +110,20 @@ graph_attr <- function(graph, name) {
 #' g
 #' plot(g)
 set_graph_attr <- function(graph, name, value) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   .Call(R_igraph_mybracket3_set, graph, igraph_t_idx_attr, igraph_attr_idx_graph, name, value)
 }
 
 #' @export
 graph.attributes <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   .Call(R_igraph_mybracket2_copy, graph, igraph_t_idx_attr, igraph_attr_idx_graph)
 }
 
 #' @export
 "graph.attributes<-" <- function(graph, value) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (!is.list(value) || (length(value) > 0 && is.null(names(value))) ||
     any(names(value) == "") || any(duplicated(names(value)))) {
     stop("Value must be a named list with unique names")
@@ -163,9 +155,7 @@ graph.attributes <- function(graph) {
 #' vertex_attr(g)
 #' plot(g)
 vertex_attr <- function(graph, name, index = V(graph)) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (missing(name)) {
     if (missing(index)) {
       vertex.attributes(graph)
@@ -246,9 +236,7 @@ set_vertex_attr <- function(graph, name, index = V(graph), value) {
 }
 
 i_set_vertex_attr <- function(graph, name, index = V(graph), value, check = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (is.null(value)) {
     return(graph)
@@ -296,9 +284,7 @@ i_set_vertex_attr <- function(graph, name, index = V(graph), value, check = TRUE
 
 #' @export
 vertex.attributes <- function(graph, index = V(graph)) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (!missing(index)) {
     index <- as.igraph.vs(graph, index)
@@ -317,9 +303,8 @@ vertex.attributes <- function(graph, index = V(graph)) {
 
 #' @export
 "vertex.attributes<-" <- function(graph, index = V(graph), value) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   if (!is.list(value) || (length(value) > 0 && is.null(names(value))) ||
     any(names(value) == "") || any(duplicated(names(value)))) {
     stop("Value must be a named list with unique names")
@@ -373,9 +358,8 @@ vertex.attributes <- function(graph, index = V(graph)) {
 #' g
 #' plot(g, edge.width = E(g)$weight)
 edge_attr <- function(graph, name, index = E(graph)) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   if (missing(name)) {
     if (missing(index)) {
       edge.attributes(graph)
@@ -456,9 +440,7 @@ set_edge_attr <- function(graph, name, index = E(graph), value) {
 }
 
 i_set_edge_attr <- function(graph, name, index = E(graph), value, check = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (is.null(value)) {
     return(graph)
@@ -506,9 +488,7 @@ i_set_edge_attr <- function(graph, name, index = E(graph), value, check = TRUE)
 
 #' @export
 edge.attributes <- function(graph, index = E(graph)) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (!missing(index)) {
     index <- as.igraph.es(graph, index)
@@ -527,9 +507,7 @@ edge.attributes <- function(graph, index = E(graph)) {
 
 #' @export
 "edge.attributes<-" <- function(graph, index = E(graph), value) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (!is.list(value) || (length(value) > 0 && is.null(names(value))) ||
     any(names(value) == "") || any(duplicated(names(value)))) {
@@ -574,9 +552,7 @@ edge.attributes <- function(graph, index = E(graph)) {
 #' g <- make_ring(10)
 #' graph_attr_names(g)
 graph_attr_names <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   res <- .Call(R_igraph_mybracket2_names, graph, igraph_t_idx_attr, igraph_attr_idx_graph)
   if (is.null(res)) {
     res <- character()
@@ -600,9 +576,8 @@ graph_attr_names <- function(graph) {
 #' vertex_attr_names(g)
 #' plot(g)
 vertex_attr_names <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   res <- .Call(R_igraph_mybracket2_names, graph, igraph_t_idx_attr, igraph_attr_idx_vertex)
 
   if (is.null(res)) {
@@ -626,9 +601,7 @@ vertex_attr_names <- function(graph) {
 #' edge_attr_names(g)
 #' plot(g)
 edge_attr_names <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   res <- .Call(R_igraph_mybracket2_names, graph, igraph_t_idx_attr, igraph_attr_idx_edge)
   if (is.null(res)) {
     res <- character()
@@ -652,9 +625,8 @@ edge_attr_names <- function(graph) {
 #' g2 <- delete_graph_attr(g, "name")
 #' graph_attr_names(g2)
 delete_graph_attr <- function(graph, name) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   name <- as.character(name)
   if (!name %in% graph_attr_names(graph)) {
     stop("No such graph attribute: ", name)
@@ -683,9 +655,8 @@ delete_graph_attr <- function(graph, name) {
 #' g2 <- delete_vertex_attr(g, "name")
 #' vertex_attr_names(g2)
 delete_vertex_attr <- function(graph, name) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   name <- as.character(name)
   if (!name %in% vertex_attr_names(graph)) {
     stop("No such vertex attribute: ", name)
@@ -714,9 +685,8 @@ delete_vertex_attr <- function(graph, name) {
 #' g2 <- delete_edge_attr(g, "name")
 #' edge_attr_names(g2)
 delete_edge_attr <- function(graph, name) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   name <- as.character(name)
   if (!name %in% edge_attr_names(graph)) {
     stop("No such edge attribute: ", name)
@@ -764,9 +734,8 @@ delete_edge_attr <- function(graph, name) {
 #' neighbors(g, "a")
 #'
 is_named <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   "name" %in% vertex_attr_names(graph)
 }
 
@@ -802,9 +771,8 @@ is_named <- function(graph) {
 #' shortest_paths(g, 8, 2)
 #'
 is_weighted <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   "weight" %in% edge_attr_names(graph)
 }
 
@@ -812,9 +780,8 @@ is_weighted <- function(graph) {
 #' @family bipartite
 #' @export
 is_bipartite <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   "type" %in% vertex_attr_names(graph)
 }
 
diff --git a/R/basic.R b/R/basic.R
index 3d207c2333d..2bea28c03dc 100644
--- a/R/basic.R
+++ b/R/basic.R
@@ -46,9 +46,7 @@ get.edge <- function(graph, id) {
     "'ends' instead."
   ))
 
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   id <- as.numeric(id)
   ec <- ecount(graph)
diff --git a/R/bipartite.R b/R/bipartite.R
index e7b91b61cfd..5afcda66f9d 100644
--- a/R/bipartite.R
+++ b/R/bipartite.R
@@ -94,9 +94,7 @@ bipartite_projection <- function(graph, types = NULL,
                                  which = c("both", "true", "false"),
                                  remove.type = TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   types <- handle_vertex_type_arg(types, graph)
   if (!is.null(probe1)) {
     probe1 <- as.igraph.vs(graph, probe1) - 1
diff --git a/R/centrality.R b/R/centrality.R
index 7c7594a3ede..bec35ff9f14 100644
--- a/R/centrality.R
+++ b/R/centrality.R
@@ -117,9 +117,8 @@ estimate_betweenness <- function(graph, vids = V(graph), directed = TRUE, cutoff
 #'
 betweenness <- function(graph, v = V(graph), directed = TRUE, weights = NULL,
                         nobigint = TRUE, normalized = FALSE, cutoff = -1) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   v <- as.igraph.vs(graph, v)
   directed <- as.logical(directed)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
@@ -161,9 +160,8 @@ betweenness <- function(graph, v = V(graph), directed = TRUE, weights = NULL,
 edge_betweenness <- function(graph, e = E(graph),
                              directed = TRUE, weights = NULL, cutoff = -1) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   e <- as.igraph.es(graph, e)
   directed <- as.logical(directed)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
@@ -262,9 +260,8 @@ closeness <- function(graph, vids = V(graph),
                       mode = c("out", "in", "all", "total"), weights = NULL,
                       normalized = FALSE, cutoff = -1) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   vids <- as.igraph.vs(graph, vids)
   mode <- switch(igraph.match.arg(mode),
     "out" = 1,
@@ -1062,9 +1059,7 @@ harmonic_centrality <- harmonic_centrality_cutoff_impl
 bonpow.dense <- function(graph, nodes = V(graph),
                          loops = FALSE, exponent = 1,
                          rescale = FALSE, tol = 1e-7) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   d <- as_adj(graph)
   if (!loops) {
@@ -1240,9 +1235,7 @@ power_centrality <- function(graph, nodes = V(graph),
 alpha.centrality.dense <- function(graph, nodes = V(graph), alpha = 1,
                                    loops = FALSE, exo = 1, weights = NULL,
                                    tol = 1e-7) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   exo <- rep(exo, length.out = vcount(graph))
   exo <- matrix(exo, ncol = 1)
@@ -1280,9 +1273,7 @@ alpha.centrality.dense <- function(graph, nodes = V(graph), alpha = 1,
 alpha.centrality.sparse <- function(graph, nodes = V(graph), alpha = 1,
                                     loops = FALSE, exo = 1, weights = NULL,
                                     tol = 1e-7) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   vc <- vcount(graph)
 
diff --git a/R/centralization.R b/R/centralization.R
index 0166f85c470..4e07a41d908 100644
--- a/R/centralization.R
+++ b/R/centralization.R
@@ -153,9 +153,8 @@ centr_degree_tmax <- function(graph = NULL, nodes = 0, mode = c("all", "out", "i
     warning("centr_degree_tmax() will require an explicit value for its 'loops' argument from igraph 1.4.0. Assuming FALSE now.")
   }
   # Argument checks
-  if (!is.null(graph) && !is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph, optional = TRUE)
+
   nodes <- as.integer(nodes)
   mode <- switch(igraph.match.arg(mode),
     "out" = 1,
@@ -207,9 +206,8 @@ centr_degree_tmax <- function(graph = NULL, nodes = 0, mode = c("all", "out", "i
 #' centr_eigen(g, directed = FALSE)$centralization
 centr_betw <- function(graph, directed = TRUE, nobigint = TRUE, normalized = TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   directed <- as.logical(directed)
   normalized <- as.logical(normalized)
 
diff --git a/R/cliques.R b/R/cliques.R
index b3f77a85c79..b06fb013cbc 100644
--- a/R/cliques.R
+++ b/R/cliques.R
@@ -108,9 +108,7 @@ largest_cliques <- largest_cliques_impl
 #' @family cliques
 #' @export
 max_cliques <- function(graph, min = NULL, max = NULL, subset = NULL, file = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (is.null(min)) {
     min <- 0
@@ -164,9 +162,7 @@ max_cliques <- function(graph, min = NULL, max = NULL, subset = NULL, file = NUL
 count_max_cliques <- function(graph, min = NULL, max = NULL,
                               subset = NULL) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (is.null(min)) {
     min <- 0
@@ -325,9 +321,7 @@ weighted_clique_num <- weighted_clique_number_impl
 #'
 #' length(maximal_ivs(g))
 ivs <- function(graph, min = NULL, max = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (is.null(min)) {
     min <- 0
@@ -354,9 +348,7 @@ ivs <- function(graph, min = NULL, max = NULL) {
 #' @family cliques
 #' @export
 largest_ivs <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   on.exit(.Call(R_igraph_finalizer))
   res <- .Call(R_igraph_largest_independent_vertex_sets, graph)
@@ -372,9 +364,7 @@ largest_ivs <- function(graph) {
 #' @family cliques
 #' @export
 maximal_ivs <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   on.exit(.Call(R_igraph_finalizer))
   res <- .Call(R_igraph_maximal_independent_vertex_sets, graph)
@@ -390,9 +380,7 @@ maximal_ivs <- function(graph) {
 #' @family cliques
 #' @export
 ivs_size <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   on.exit(.Call(R_igraph_finalizer))
   .Call(R_igraph_independence_number, graph)
diff --git a/R/cocitation.R b/R/cocitation.R
index eb500f85520..d4fc440487f 100644
--- a/R/cocitation.R
+++ b/R/cocitation.R
@@ -57,9 +57,8 @@
 #' bibcoupling(g)
 #'
 cocitation <- function(graph, v = V(graph)) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   v <- as.igraph.vs(graph, v)
   on.exit(.Call(R_igraph_finalizer))
   res <- .Call(R_igraph_cocitation, graph, v - 1)
@@ -73,9 +72,8 @@ cocitation <- function(graph, v = V(graph)) {
 #' @family cocitation
 #' @export
 bibcoupling <- function(graph, v = V(graph)) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   v <- as.igraph.vs(graph, v)
   on.exit(.Call(R_igraph_finalizer))
   res <- .Call(R_igraph_bibcoupling, graph, v - 1)
diff --git a/R/cohesive.blocks.R b/R/cohesive.blocks.R
index f8249b6fceb..fe58b9be021 100644
--- a/R/cohesive.blocks.R
+++ b/R/cohesive.blocks.R
@@ -247,9 +247,7 @@
 #'
 cohesive_blocks <- function(graph, labels = TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   on.exit(.Call(R_igraph_finalizer))
   # Function call
diff --git a/R/community.R b/R/community.R
index 0bfff1a684a..021920036e9 100644
--- a/R/community.R
+++ b/R/community.R
@@ -440,9 +440,7 @@ modularity <- function(x, ...) {
 #'
 modularity.igraph <- function(x, membership, weights = NULL, resolution = 1, directed = TRUE, ...) {
   # Argument checks
-  if (!is_igraph(x)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(x)
   if (is.null(membership) || (!is.numeric(membership) && !is.factor(membership))) {
     stop("Membership is not a numerical vector")
   }
@@ -475,9 +473,8 @@ modularity.communities <- function(x, ...) {
 #' @export
 modularity_matrix <- function(graph, membership, weights = NULL, resolution = 1, directed = TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   if (!missing(membership)) {
     warning("The membership argument is deprecated; modularity_matrix does not need it")
   }
@@ -970,9 +967,7 @@ cluster_spinglass <- function(graph, weights = NULL, vertex = NULL, spins = 25,
                               update.rule = c("config", "random", "simple"),
                               gamma = 1.0, implementation = c("orig", "neg"),
                               gamma.minus = 1.0) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -1128,9 +1123,7 @@ cluster_spinglass <- function(graph, weights = NULL, vertex = NULL, spins = 25,
 cluster_leiden <- function(graph, objective_function = c("CPM", "modularity"),
                            weights = NULL, resolution_parameter = 1, beta = 0.01,
                            initial_membership = NULL, n_iterations = 2, vertex_weights = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   # Parse objective function argument
   objective_function <- igraph.match.arg(objective_function)
@@ -1247,9 +1240,8 @@ cluster_leiden <- function(graph, objective_function = c("CPM", "modularity"),
 #' comms <- cluster_fluid_communities(g, 2)
 cluster_fluid_communities <- function(graph, no.of.communities) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   no.of.communities <- as.integer(no.of.communities)
 
   on.exit(.Call(R_igraph_finalizer))
@@ -1322,9 +1314,7 @@ cluster_fluid_communities <- function(graph, no.of.communities) {
 cluster_walktrap <- function(graph, weights = NULL, steps = 4,
                              merges = TRUE, modularity = TRUE,
                              membership = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object!")
-  }
+  ensure_igraph(graph)
 
   if (membership && !modularity) {
     modularity <- TRUE
@@ -1451,9 +1441,7 @@ cluster_edge_betweenness <- function(graph, weights = NULL,
                                      merges = TRUE, bridges = TRUE,
                                      modularity = TRUE,
                                      membership = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object!")
-  }
+  ensure_igraph(graph)
 
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -1536,9 +1524,7 @@ cluster_edge_betweenness <- function(graph, weights = NULL,
 #'
 cluster_fast_greedy <- function(graph, merges = TRUE, modularity = TRUE,
                                 membership = TRUE, weights = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -1673,9 +1659,8 @@ cluster_leading_eigen <- function(graph, steps = -1, weights = NULL,
                                   callback = NULL, extra = NULL,
                                   env = parent.frame()) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   steps <- as.integer(steps)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -1770,9 +1755,8 @@ cluster_leading_eigen <- function(graph, steps = -1, weights = NULL,
 #'
 cluster_label_prop <- function(graph, weights = NULL, initial = NULL, fixed = NULL) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
@@ -1864,9 +1848,8 @@ cluster_label_prop <- function(graph, weights = NULL, initial = NULL, fixed = NU
 #'
 cluster_louvain <- function(graph, weights = NULL, resolution = 1) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
@@ -1957,9 +1940,8 @@ cluster_louvain <- function(graph, weights = NULL, resolution = 1) {
 #' @keywords graphs
 cluster_optimal <- function(graph, weights = NULL) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
@@ -2036,9 +2018,8 @@ cluster_optimal <- function(graph, weights = NULL) {
 cluster_infomap <- function(graph, e.weights = NULL, v.weights = NULL,
                             nb.trials = 10, modularity = TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   if (is.null(e.weights) && "weight" %in% edge_attr_names(graph)) {
     e.weights <- E(graph)$weight
   }
diff --git a/R/components.R b/R/components.R
index 99a50eb4948..2ee19725a36 100644
--- a/R/components.R
+++ b/R/components.R
@@ -26,9 +26,7 @@
 #' @family components
 #' @export
 count_components <- function(graph, mode = c("weak", "strong")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
     "weak" = 1,
@@ -49,9 +47,7 @@ count_components <- function(graph, mode = c("weak", "strong")) {
 #' @importFrom graphics hist
 component_distribution <- function(graph, cumulative = FALSE, mul.size = FALSE,
                                    ...) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   cs <- components(graph, ...)$csize
   hi <- hist(cs, -1:max(cs), plot = FALSE)$density
@@ -103,9 +99,7 @@ component_distribution <- function(graph, cumulative = FALSE, mul.size = FALSE,
 #'
 decompose <- function(graph, mode = c("weak", "strong"), max.comps = NA,
                       min.vertices = 0) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
     "weak" = 1,
diff --git a/R/conversion.R b/R/conversion.R
index b3d8af59d10..f3366820fee 100644
--- a/R/conversion.R
+++ b/R/conversion.R
@@ -21,9 +21,7 @@
 
 get.adjacency.dense <- function(graph, type = c("both", "upper", "lower"),
                                 attr = NULL, edges = FALSE, names = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   type <- igraph.match.arg(type)
   type <- switch(type,
@@ -96,9 +94,7 @@ get.adjacency.dense <- function(graph, type = c("both", "upper", "lower"),
 
 get.adjacency.sparse <- function(graph, type = c("both", "upper", "lower"),
                                  attr = NULL, edges = FALSE, names = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   type <- igraph.match.arg(type)
 
@@ -212,9 +208,7 @@ get.adjacency.sparse <- function(graph, type = c("both", "upper", "lower"),
 as_adjacency_matrix <- function(graph, type = c("both", "upper", "lower"),
                                 attr = NULL, edges = FALSE, names = TRUE,
                                 sparse = igraph_opt("sparsematrices")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (!missing(edges)) {
     warning("The `edges` argument of `as_adjacency_matrix` is deprecated; it will be removed in igraph 1.4.0")
@@ -258,9 +252,7 @@ as_adj <- as_adjacency_matrix
 #' @family conversion
 #' @export
 as_edgelist <- function(graph, names = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   on.exit(.Call(R_igraph_finalizer))
   res <- matrix(.Call(R_igraph_get_edgelist, graph, TRUE),
     ncol = 2
@@ -366,9 +358,7 @@ as.directed <- to_directed_impl
 #' @export
 as.undirected <- function(graph, mode = c("collapse", "each", "mutual"), edge.attr.comb = igraph_opt("edge.attr.comb")) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- switch(igraph.match.arg(mode),
     "collapse" = 1,
     "each" = 0,
@@ -429,9 +419,7 @@ as_adj_list <- function(graph,
                         mode = c("all", "out", "in", "total"),
                         loops = c("twice", "once", "ignore"),
                         multiple = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   mode <- igraph.match.arg(mode)
   mode <- as.numeric(switch(mode,
@@ -469,9 +457,7 @@ as_adj_list <- function(graph,
 as_adj_edge_list <- function(graph,
                              mode = c("all", "out", "in", "total"),
                              loops = c("twice", "once", "ignore")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   mode <- igraph.match.arg(mode)
   mode <- as.numeric(switch(mode,
@@ -639,9 +625,7 @@ graph_from_graphnel <- function(graphNEL, name = TRUE, weight = TRUE,
 #' @family conversion
 #' @export
 as_graphnel <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not an igraph graph")
-  }
+  ensure_igraph(graph)
 
   if (any_multiple(graph)) {
     stop("multiple edges are not supported in graphNEL graphs")
@@ -862,9 +846,7 @@ get.incidence.sparse <- function(graph, types, names, attr) {
 as_incidence_matrix <- function(graph, types = NULL, attr = NULL,
                                 names = TRUE, sparse = FALSE) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   types <- handle_vertex_type_arg(types, graph)
 
   names <- as.logical(names)
@@ -884,9 +866,7 @@ as_incidence_matrix <- function(graph, types = NULL, attr = NULL,
 #' @family conversion
 #' @export
 as_data_frame <- function(x, what = c("edges", "vertices", "both")) {
-  if (!is_igraph(x)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(x)
   what <- igraph.match.arg(what)
 
   if (what %in% c("vertices", "both")) {
@@ -998,9 +978,7 @@ graph_from_adj_list <- adjlist_impl
 #' )
 #' as_long_data_frame(g)
 as_long_data_frame <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   ver <- .Call(R_igraph_mybracket2, graph, igraph_t_idx_attr, igraph_attr_idx_vertex)
   class(ver) <- "data.frame"
diff --git a/R/decomposition.R b/R/decomposition.R
index 0b4eb4cf253..4f93bbc857b 100644
--- a/R/decomposition.R
+++ b/R/decomposition.R
@@ -87,9 +87,7 @@
 #'
 is_chordal <- function(graph, alpha = NULL, alpham1 = NULL,
                        fillin = FALSE, newgraph = FALSE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (!is.null(alpha)) {
     alpha <- as.numeric(alpha) - 1
   }
diff --git a/R/flow.R b/R/flow.R
index 9d4af216ed6..c30d71c3249 100644
--- a/R/flow.R
+++ b/R/flow.R
@@ -80,9 +80,7 @@
 #' @family flow
 #' @export
 min_cut <- function(graph, source = NULL, target = NULL, capacity = NULL, value.only = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (is.null(capacity)) {
     if ("capacity" %in% edge_attr_names(graph)) {
       capacity <- E(graph)$capacity
@@ -223,9 +221,7 @@ min_cut <- function(graph, source = NULL, target = NULL, capacity = NULL, value.
 #' cohesion(g)
 #'
 vertex_connectivity <- function(graph, source = NULL, target = NULL, checks = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (length(source) == 0) {
     source <- NULL
@@ -315,9 +311,7 @@ vertex_connectivity <- function(graph, source = NULL, target = NULL, checks = TR
 #' adhesion(g)
 #'
 edge_connectivity <- function(graph, source = NULL, target = NULL, checks = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (length(source) == 0) {
     source <- NULL
@@ -343,9 +337,7 @@ edge_connectivity <- function(graph, source = NULL, target = NULL, checks = TRUE
 #' @family flow
 #' @export
 edge_disjoint_paths <- function(graph, source, target) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (length(source) == 0) {
     source <- NULL
@@ -364,9 +356,7 @@ edge_disjoint_paths <- function(graph, source, target) {
 #' @family flow
 #' @export
 vertex_disjoint_paths <- function(graph, source = NULL, target = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (length(source) == 0) {
     source <- NULL
@@ -385,9 +375,7 @@ vertex_disjoint_paths <- function(graph, source = NULL, target = NULL) {
 #' @family flow
 #' @export
 adhesion <- function(graph, checks = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   on.exit(.Call(R_igraph_finalizer))
   .Call(R_igraph_adhesion, graph, as.logical(checks))
@@ -398,9 +386,7 @@ adhesion <- function(graph, checks = TRUE) {
 #' @family flow
 #' @export
 cohesion.igraph <- function(x, checks = TRUE, ...) {
-  if (!is_igraph(x)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(x)
 
   on.exit(.Call(R_igraph_finalizer))
   .Call(R_igraph_cohesion, x, as.logical(checks))
@@ -555,9 +541,7 @@ st_min_cuts <- all_st_mincuts_impl
 #' @export
 dominator_tree <- function(graph, root, mode = c("out", "in", "all", "total")) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+ ensure_igraph(graph)
   root <- as.igraph.vs(graph, root)
   if (length(root) == 0) {
     stop("No vertex was specified")
diff --git a/R/foreign.R b/R/foreign.R
index 7bbf8c16391..73466162546 100644
--- a/R/foreign.R
+++ b/R/foreign.R
@@ -178,9 +178,7 @@ write_graph <- function(graph, file,
                           "graphml", "dimacs", "gml", "dot", "leda"
                         ),
                         ...) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (!is.character(file) || length(grep("://", file, fixed = TRUE)) > 0 ||
     length(grep("~", file, fixed = TRUE)) > 0) {
     tmpfile <- TRUE
diff --git a/R/games.R b/R/games.R
index 18f1bd87417..0f1fd85a3ee 100644
--- a/R/games.R
+++ b/R/games.R
@@ -1073,9 +1073,7 @@ asym_pref <- function(...) constructor_spec(sample_asym_pref, ...)
 #' @export
 #' @family functions for manipulating graph structure
 connect <- function(graph, order, mode = c("all", "out", "in", "total")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
     "out" = 1,
diff --git a/R/glet.R b/R/glet.R
index e1ce4430c40..9dfc3690516 100644
--- a/R/glet.R
+++ b/R/glet.R
@@ -80,9 +80,7 @@
 #' @export
 graphlet_basis <- function(graph, weights = NULL) {
   ## Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
@@ -109,9 +107,7 @@ graphlet_basis <- function(graph, weights = NULL) {
 graphlet_proj <- function(graph, weights = NULL, cliques, niter = 1000,
                           Mu = rep(1, length(cliques))) {
   # Argument checks
-  if (!is.igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
   }
diff --git a/R/hrg.R b/R/hrg.R
index f27bc403734..b5a8e7108c8 100644
--- a/R/hrg.R
+++ b/R/hrg.R
@@ -114,9 +114,7 @@ NULL
 #' @family hierarchical random graph functions
 fit_hrg <- function(graph, hrg = NULL, start = FALSE, steps = 0) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (is.null(hrg)) {
     hrg <- list(
       left = c(), right = c(), prob = c(), edges = c(),
@@ -282,9 +280,7 @@ sample_hrg <- hrg_game_impl
 predict_edges <- function(graph, hrg = NULL, start = FALSE, num.samples = 10000,
                           num.bins = 25) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (is.null(hrg)) {
     hrg <- list(
       left = c(), right = c(), prob = c(), edges = c(),
diff --git a/R/interface.R b/R/interface.R
index 77df4ef034d..e52261c6746 100644
--- a/R/interface.R
+++ b/R/interface.R
@@ -62,9 +62,7 @@
 #' E(g)[[]]
 #' plot(g)
 add_edges <- function(graph, edges, ..., attr = list()) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   attrs <- list(...)
   attrs <- append(attrs, attr)
@@ -128,9 +126,7 @@ add_edges <- function(graph, edges, ..., attr = list()) {
 #' V(g)[[]]
 #' plot(g)
 add_vertices <- function(graph, nv, ..., attr = list()) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   attrs <- list(...)
   attrs <- append(attrs, attr)
@@ -186,9 +182,8 @@ add_vertices <- function(graph, nv, ..., attr = list()) {
 #' g <- delete_edges(g, get.edge.ids(g, c(1, 5, 4, 5)))
 #' g
 delete_edges <- function(graph, edges) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   on.exit(.Call(R_igraph_finalizer))
   .Call(R_igraph_delete_edges, graph, as.igraph.es(graph, edges) - 1)
 }
@@ -214,9 +209,8 @@ delete_edges <- function(graph, edges) {
 #' g2
 #' V(g2)
 delete_vertices <- function(graph, v) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   on.exit(.Call(R_igraph_finalizer))
   .Call(R_igraph_delete_vertices, graph, as.igraph.vs(graph, v) - 1)
 }
@@ -246,9 +240,8 @@ delete_vertices <- function(graph, v) {
 #'   vapply(gsize, 0) %>%
 #'   hist()
 gsize <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   on.exit(.Call(R_igraph_finalizer))
   .Call(R_igraph_ecount, graph)
 }
@@ -277,9 +270,7 @@ ecount <- gsize
 #' n34 <- neighbors(g, 34)
 #' intersection(n1, n34)
 neighbors <- function(graph, v, mode = c("out", "in", "all", "total")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (is.character(mode)) {
     mode <- igraph.match.arg(mode)
     mode <- switch(mode,
@@ -319,9 +310,7 @@ neighbors <- function(graph, v, mode = c("out", "in", "all", "total")) {
 #' incident(g, 1)
 #' incident(g, 34)
 incident <- function(graph, v, mode = c("all", "out", "in", "total")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (is_directed(graph)) {
     mode <- igraph.match.arg(mode)
     mode <- switch(mode,
@@ -361,9 +350,8 @@ incident <- function(graph, v, mode = c("all", "out", "in", "total")) {
 #' g2 <- make_ring(10, directed = TRUE)
 #' is_directed(g2)
 is_directed <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   on.exit(.Call(R_igraph_finalizer))
   .Call(R_igraph_is_directed, graph)
 }
@@ -385,9 +373,7 @@ is_directed <- function(graph) {
 #' g <- make_ring(5)
 #' ends(g, E(g))
 ends <- function(graph, es, names = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   es2 <- as.igraph.es(graph, na.omit(es)) - 1
   res <- matrix(NA_integer_, ncol = length(es), nrow = 2)
@@ -470,9 +456,8 @@ get.edges <- function(graph, es) {
 #' E(g)[eim]
 #'
 get.edge.ids <- function(graph, vp, directed = TRUE, error = FALSE, multi = FALSE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   on.exit(.Call(R_igraph_finalizer))
   .Call(
     R_igraph_get_eids, graph, as.igraph.vs(graph, vp) - 1,
@@ -520,7 +505,7 @@ vcount <- vcount_impl
 #' adjacent_vertices(g, c(1, 34))
 adjacent_vertices <- function(graph, v,
                               mode = c("out", "in", "all", "total")) {
-  if (!is_igraph(graph)) stop("Not a graph object")
+  ensure_igraph(graph)
 
   vv <- as.igraph.vs(graph, v) - 1
   mode <- switch(match.arg(mode),
@@ -563,7 +548,7 @@ adjacent_vertices <- function(graph, v,
 #' incident_edges(g, c(1, 34))
 incident_edges <- function(graph, v,
                            mode = c("out", "in", "all", "total")) {
-  if (!is_igraph(graph)) stop("Not a graph object")
+  ensure_igraph(graph)
 
   vv <- as.igraph.vs(graph, v) - 1
   mode <- switch(match.arg(mode),
diff --git a/R/iterators.R b/R/iterators.R
index f4ed399e6b1..a1793e178b7 100644
--- a/R/iterators.R
+++ b/R/iterators.R
@@ -205,9 +205,7 @@ set_complete_iterator <- function(x, value = TRUE) {
 #'   set_vertex_attr("name", value = letters[1:10])
 #' V(g2)
 V <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   update_vs_ref(graph)
 
@@ -304,9 +302,7 @@ unsafe_create_es <- function(graph, idx, es = NULL) {
 #'   set_vertex_attr("name", value = letters[1:10])
 #' E(g2)
 E <- function(graph, P = NULL, path = NULL, directed = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   update_es_ref(graph)
 
@@ -1182,9 +1178,7 @@ simple_es_index <- function(x, i, na_ok = FALSE) {
 #' @name igraph-vs-attributes
 #' @export
 `V<-` <- function(x, value) {
-  if (!is_igraph(x)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(x)
   if (!"name" %in% names(attributes(value)) ||
     !"value" %in% names(attributes(value))) {
     stop("invalid indexing")
@@ -1203,9 +1197,7 @@ simple_es_index <- function(x, i, na_ok = FALSE) {
 #' @name igraph-es-attributes
 #' @export
 `E<-` <- function(x, path = NULL, P = NULL, directed = NULL, value) {
-  if (!is_igraph(x)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(x)
   if (!"name" %in% names(attributes(value)) ||
     !"value" %in% names(attributes(value))) {
     stop("invalid indexing")
diff --git a/R/layout.R b/R/layout.R
index cea5e848eed..8466096d393 100644
--- a/R/layout.R
+++ b/R/layout.R
@@ -283,9 +283,7 @@ normalize <- function(xmin = -1, xmax = 1, ymin = xmin, ymax = xmax,
 layout_as_bipartite <- function(graph, types = NULL, hgap = 1, vgap = 1,
                                 maxiter = 100) {
   ## Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   types <- handle_vertex_type_arg(types, graph)
   hgap <- as.numeric(hgap)
   vgap <- as.numeric(vgap)
@@ -341,9 +339,7 @@ as_bipartite <- function(...) layout_spec(layout_as_bipartite, ...)
 #' layout_(g, as_star())
 layout_as_star <- function(graph, center = V(graph)[1], order = NULL) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (vcount(graph) == 0) {
     # Any other layout will do so just pick one that supports graphs with no
     # vertices
@@ -431,9 +427,7 @@ as_star <- function(...) layout_spec(layout_as_star, ...)
 layout_as_tree <- function(graph, root = numeric(), circular = FALSE,
                            rootlevel = numeric(), mode = c("out", "in", "all"),
                            flip.y = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   root <- as.igraph.vs(graph, root) - 1
   circular <- as.logical(circular)
   rootlevel <- as.double(rootlevel)
@@ -503,9 +497,7 @@ layout.reingold.tilford <- function(..., params = list()) {
 #' V(karate)$shape <- "none"
 #' plot(karate, layout = coords)
 layout_in_circle <- function(graph, order = V(graph)) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   order <- as.igraph.vs(graph, order) - 1L
   on.exit(.Call(R_igraph_finalizer))
   .Call(R_igraph_layout_circle, graph, order)
@@ -666,9 +658,7 @@ nicely <- function(...) layout_spec(layout_nicely, ...)
 #' }
 layout_on_grid <- function(graph, width = 0, height = 0, dim = 2) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   width <- as.integer(width)
   dim <- as.integer(dim)
   stopifnot(dim == 2 || dim == 3)
@@ -703,9 +693,7 @@ layout.grid.3d <- function(graph, width = 0, height = 0) {
     "igraph 0.8.0, please use layout_on_grid instead"
   ))
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   width <- as.integer(width)
   height <- as.integer(height)
 
@@ -738,9 +726,8 @@ layout.grid.3d <- function(graph, width = 0, height = 0) {
 #' @export
 #' @family graph layouts
 layout_on_sphere <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   on.exit(.Call(R_igraph_finalizer))
   .Call(R_igraph_layout_sphere, graph)
 }
@@ -778,9 +765,7 @@ layout.sphere <- function(..., params = list()) {
 #' @export
 #' @family graph layouts
 layout_randomly <- function(graph, dim = 2) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (dim == 2) {
     on.exit(.Call(R_igraph_finalizer))
     .Call(R_igraph_layout_random, graph)
@@ -924,9 +909,7 @@ layout_with_dh <- function(graph, coords = NULL, maxiter = 10,
                            weight.edge.crossings = 1.0 - sqrt(edge_density(graph)),
                            weight.node.edge.dist = 0.2 * (1 - edge_density(graph))) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (!is.null(coords)) {
     coords <- as.matrix(structure(as.double(coords), dim = dim(coords)))
     use.seed <- TRUE
@@ -1049,9 +1032,7 @@ layout_with_fr <- function(graph, coords = NULL, dim = 2,
                            minz = NULL, maxz = NULL,
                            coolexp, maxdelta, area, repulserad, maxiter) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (!is.null(coords)) {
     coords <- as.matrix(structure(as.double(coords), dim = dim(coords)))
   }
@@ -1174,9 +1155,7 @@ layout_with_gem <- function(graph, coords = NULL, maxiter = 40 * vcount(graph)^2
                             temp.max = max(vcount(graph), 1), temp.min = 1 / 10,
                             temp.init = sqrt(max(vcount(graph), 1))) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (!is.null(coords)) {
     coords <- as.matrix(structure(as.double(coords), dim = dim(coords)))
     use.seed <- TRUE
@@ -1256,9 +1235,7 @@ with_gem <- function(...) layout_spec(layout_with_gem, ...)
 layout_with_graphopt <- function(graph, start = NULL, niter = 500, charge = 0.001,
                                  mass = 30, spring.length = 0, spring.constant = 1,
                                  max.sa.movement = 5) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (!is.null(start)) {
     start <- structure(as.numeric(start), dim = dim(start))
   }
@@ -1358,9 +1335,7 @@ layout_with_kk <- function(graph, coords = NULL, dim = 2,
   }
   if (!missing(start)) coords <- start
 
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (!is.null(coords)) {
     coords <- as.matrix(structure(as.double(coords), dim = dim(coords)))
   }
@@ -1466,9 +1441,7 @@ layout_with_lgl <- function(graph, maxiter = 150, maxdelta = vcount(graph),
                             area = vcount(graph)^2, coolexp = 1.5,
                             repulserad = area * vcount(graph),
                             cellsize = sqrt(sqrt(area)), root = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (is.null(root)) {
     root <- -1
   } else {
@@ -1514,7 +1487,7 @@ layout.lgl <- function(..., params = list()) {
 #'
 #' Warning: If the graph is symmetric to the exchange of two vertices (as is the
 #' case with leaves of a tree connecting to the same parent), classical
-#' multidimensional scaling may assign the same coordinates to these vertices. 
+#' multidimensional scaling may assign the same coordinates to these vertices.
 #'
 #' This function generates the layout separately for each graph component and
 #' then merges them via [merge_coords()].
@@ -1546,9 +1519,7 @@ layout.lgl <- function(..., params = list()) {
 layout_with_mds <- function(graph, dist = NULL, dim = 2,
                             options = arpack_defaults) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (!is.null(dist)) dist <- structure(as.double(dist), dim = dim(dist))
   dim <- as.integer(dim)
 
@@ -1775,9 +1746,7 @@ layout_with_sugiyama <- function(graph, layers = NULL, hgap = 1, vgap = 1,
                                  maxiter = 100, weights = NULL,
                                  attributes = c("default", "all", "none")) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   if (!is.null(layers)) layers <- as.numeric(layers) - 1
   hgap <- as.numeric(hgap)
   vgap <- as.numeric(vgap)
@@ -1955,9 +1924,7 @@ with_sugiyama <- function(...) layout_spec(layout_with_sugiyama, ...)
 #' g <- disjoint_union(graphs)
 #' plot(g, layout = lay, vertex.size = 3, labels = NA, edge.color = "black")
 merge_coords <- function(graphs, layouts, method = "dla") {
-  if (!all(sapply(graphs, is_igraph))) {
-    stop("Not a graph object")
-  }
+  lapply(graphs, ensure_igraph)
   if (method == "dla") {
     on.exit(.Call(R_igraph_finalizer))
     res <- .Call(
@@ -2033,9 +2000,7 @@ norm_coords <- function(layout, xmin = -1, xmax = 1, ymin = -1, ymax = 1,
 #' @param graph The input graph.
 #' @export
 layout_components <- function(graph, layout = layout_with_kk, ...) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   V(graph)$id <- seq(vcount(graph))
   gl <- decompose(graph)
diff --git a/R/layout_drl.R b/R/layout_drl.R
index 5c4d19dd718..f3ab659caf5 100644
--- a/R/layout_drl.R
+++ b/R/layout_drl.R
@@ -91,9 +91,7 @@ layout_with_drl <- function(graph, use.seed = FALSE,
                             weights = NULL,
                             fixed = NULL,
                             dim = 2) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (dim != 2 && dim != 3) {
     stop("`dim' must be 2 or 3")
diff --git a/R/make.R b/R/make.R
index 7e450fdf11d..eb0f0f21fab 100644
--- a/R/make.R
+++ b/R/make.R
@@ -1382,9 +1382,7 @@ chordal_ring <- function(...) constructor_spec(make_chordal_ring, ...)
 #'
 #' @export
 make_line_graph <- function(graph) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   on.exit(.Call(R_igraph_finalizer))
   res <- .Call(R_igraph_linegraph, graph)
diff --git a/R/minimum.spanning.tree.R b/R/minimum.spanning.tree.R
index 1fb9b0c0c2b..d98e51d57cd 100644
--- a/R/minimum.spanning.tree.R
+++ b/R/minimum.spanning.tree.R
@@ -64,9 +64,7 @@
 #'
 mst <- function(graph, weights = NULL,
                 algorithm = NULL, ...) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (is.null(algorithm)) {
     if (!is.null(weights) || "weight" %in% edge_attr_names(graph)) {
diff --git a/R/motifs.R b/R/motifs.R
index 99f286f9398..bcb550d1e40 100644
--- a/R/motifs.R
+++ b/R/motifs.R
@@ -52,9 +52,7 @@
 #' count_motifs(g, 3)
 #' sample_motifs(g, 3)
 motifs <- function(graph, size = 3, cut.prob = rep(0, size)) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   cut.prob <- as.numeric(cut.prob)
   if (length(cut.prob) != size) {
     cut.prob <- c(
@@ -98,9 +96,7 @@ motifs <- function(graph, size = 3, cut.prob = rep(0, size)) {
 #' count_motifs(g, 3)
 #' sample_motifs(g, 3)
 count_motifs <- function(graph, size = 3, cut.prob = rep(0, size)) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   cut.prob <- as.numeric(cut.prob)
   if (length(cut.prob) != size) {
     cut.prob <- c(
@@ -149,9 +145,7 @@ count_motifs <- function(graph, size = 3, cut.prob = rep(0, size)) {
 #' sample_motifs(g, 3)
 sample_motifs <- function(graph, size = 3, cut.prob = rep(0, size),
                           sample.size = vcount(graph) / 10, sample = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   cut.prob <- as.numeric(cut.prob)
   if (length(cut.prob) != size) {
     cut.prob <- c(
diff --git a/R/operators.R b/R/operators.R
index 55c66b71c7d..72ba77f8eb7 100644
--- a/R/operators.R
+++ b/R/operators.R
@@ -116,9 +116,7 @@ disjoint_union <- function(...) {
   graphs <- unlist(recursive = FALSE, lapply(list(...), function(l) {
     if (is_igraph(l)) list(l) else l
   }))
-  if (!all(sapply(graphs, is_igraph))) {
-    stop("Not a graph object")
-  }
+  lapply(graphs, ensure_igraph)
 
   on.exit(.Call(R_igraph_finalizer))
   res <- .Call(R_igraph_disjoint_union, graphs)
@@ -187,9 +185,7 @@ disjoint_union <- function(...) {
   graphs <- unlist(recursive = FALSE, lapply(list(...), function(l) {
     if (is_igraph(l)) list(l) else l
   }))
-  if (!all(sapply(graphs, is_igraph))) {
-    stop("Not a graph object")
-  }
+  lapply(graphs, ensure_igraph)
   if (byname != "auto" && !is.logical(byname)) {
     stop("`bynam' must be \"auto\", or logical")
   }
@@ -524,9 +520,8 @@ difference <- function(...) {
 #' print_all(G)
 #' plot(G, layout = layout_nicely(wheel))
 difference.igraph <- function(big, small, byname = "auto", ...) {
-  if (!is_igraph(big) || !is_igraph(small)) {
-    stop("argument is not a graph")
-  }
+  ensure_igraph(big)
+  ensure_igraph(small)
   if (byname != "auto" && !is.logical(byname)) {
     stop("`bynam' must be \"auto\", or logical")
   }
@@ -604,9 +599,8 @@ difference.igraph <- function(big, small, byname = "auto", ...) {
 #' graph.isomorphic(gu, make_full_graph(vcount(g)))
 #'
 complementer <- function(graph, loops = FALSE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   on.exit(.Call(R_igraph_finalizer))
   .Call(R_igraph_complementer, graph, as.logical(loops))
 }
@@ -675,9 +669,8 @@ complementer <- function(graph, loops = FALSE) {
 #' print_all(simplify(gc))
 #'
 compose <- function(g1, g2, byname = "auto") {
-  if (!is_igraph(g1) || !is_igraph(g2)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(g1)
+  ensure_igraph(g2)
 
   if (byname != "auto" && !is.logical(byname)) {
     stop("`byname' must be \"auto\", or logical")
diff --git a/R/paths.R b/R/paths.R
index eda1d2e2c76..f5136e23809 100644
--- a/R/paths.R
+++ b/R/paths.R
@@ -59,7 +59,7 @@ all_simple_paths <- function(graph, from, to = V(graph),
                              mode = c("out", "in", "all", "total"),
                              cutoff = -1) {
   ## Argument checks
-  if (!is_igraph(graph)) stop("Not a graph object")
+  ensure_igraph(graph)
   from <- as.igraph.vs(graph, from)
   to <- as.igraph.vs(graph, to)
   mode <- switch(igraph.match.arg(mode),
diff --git a/R/plot.R b/R/plot.R
index 0911eed5741..7dfd423f01a 100644
--- a/R/plot.R
+++ b/R/plot.R
@@ -92,9 +92,7 @@ plot.igraph <- function(x,
                         mark.expand = 15, loop.size = 1,
                         ...) {
   graph <- x
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   vc <- vcount(graph)
 
@@ -543,9 +541,7 @@ rglplot <- function(x, ...) {
 #' @export
 rglplot.igraph <- function(x, ...) {
   graph <- x
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   create.edge <- function(v1, v2, r1, r2, ec, ew, am, as) {
     ## these could also be parameters:
diff --git a/R/print.R b/R/print.R
index b1f53f18150..133afe47ffb 100644
--- a/R/print.R
+++ b/R/print.R
@@ -56,9 +56,7 @@
 }
 
 .print.header <- function(object, id = igraph_opt("print.id")) {
-  if (!is_igraph(object)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(object)
 
   title <- paste0(
     "IGRAPH ",
@@ -525,9 +523,7 @@ print.igraph <- function(x, full = igraph_opt("print.full"),
                          names = TRUE, max.lines = igraph_opt("auto.print.lines"),
                          id = igraph_opt("print.id"),
                          ...) {
-  if (!is_igraph(x)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(x)
 
   head_lines <- .print.header(x, id)
   if (is.logical(full) && full) {
diff --git a/R/rewire.R b/R/rewire.R
index 4b1e63394ca..cbb465af24b 100644
--- a/R/rewire.R
+++ b/R/rewire.R
@@ -82,9 +82,7 @@ keeping_degseq <- function(loops = FALSE, niter = 100) {
 }
 
 rewire_keeping_degseq <- function(graph, loops, niter) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   loops <- as.logical(loops)
   mode <- if (loops) 1 else 0
@@ -154,9 +152,8 @@ each_edge <- function(prob, loops = FALSE, multiple = FALSE, mode = c("all", "ou
 }
 
 rewire_each_edge <- function(graph, prob, loops, multiple) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   on.exit(.Call(R_igraph_finalizer))
   .Call(
     R_igraph_rewire_edges, graph, as.numeric(prob), as.logical(loops),
@@ -165,9 +162,8 @@ rewire_each_edge <- function(graph, prob, loops, multiple) {
 }
 
 rewire_each_directed_edge <- function(graph, prob, loops, mode) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   on.exit(.Call(R_igraph_finalizer))
   .Call(
     R_igraph_rewire_directed_edges, graph, as.numeric(prob), as.logical(loops),
diff --git a/R/scg.R b/R/scg.R
index 76bd4baa821..949fa186a1b 100644
--- a/R/scg.R
+++ b/R/scg.R
@@ -105,9 +105,7 @@ NULL
 #'
 stochastic_matrix <- function(graph, column.wise = FALSE,
                               sparse = igraph_opt("sparsematrices")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   column.wise <- as.logical(column.wise)
   if (length(column.wise) != 1) {
diff --git a/R/simple.R b/R/simple.R
index 7a9e4455239..6fea8448c8b 100644
--- a/R/simple.R
+++ b/R/simple.R
@@ -87,9 +87,7 @@ is_simple <- is_simple_impl
 #' @rdname simplify
 simplify_and_colorize <- function(graph) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   on.exit(.Call(R_igraph_finalizer))
   # Function call
diff --git a/R/structural.properties.R b/R/structural.properties.R
index f3cc0ba1e08..5f7d68acb6e 100644
--- a/R/structural.properties.R
+++ b/R/structural.properties.R
@@ -78,9 +78,7 @@
 #' get_diameter(g, weights = NA)
 #'
 diameter <- function(graph, directed = TRUE, unconnected = TRUE, weights = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -102,9 +100,7 @@ diameter <- function(graph, directed = TRUE, unconnected = TRUE, weights = NULL)
 #' @export
 get_diameter <- function(graph, directed = TRUE, unconnected = TRUE,
                          weights = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -132,9 +128,7 @@ get_diameter <- function(graph, directed = TRUE, unconnected = TRUE,
 #' @export
 farthest_vertices <- function(graph, directed = TRUE, unconnected = TRUE,
                               weights = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -204,9 +198,7 @@ mean_distance <- average_path_length_dijkstra_impl
 degree <- function(graph, v = V(graph),
                    mode = c("all", "out", "in", "total"), loops = TRUE,
                    normalized = FALSE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   v <- as.igraph.vs(graph, v)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
@@ -237,9 +229,7 @@ degree <- function(graph, v = V(graph),
 #' @export
 #' @importFrom graphics hist
 degree_distribution <- function(graph, cumulative = FALSE, ...) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   cs <- degree(graph, ...)
   hi <- hist(cs, -1:max(cs), plot = FALSE)$density
   if (!cumulative) {
@@ -431,9 +421,7 @@ distances <- function(graph, v = V(graph), to = V(graph),
                         "automatic", "unweighted", "dijkstra",
                         "bellman-ford", "johnson"
                       )) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   # make sure that the lower-level function in C gets mode == "out"
   # unconditionally when the graph is undirected; this is used for
@@ -519,9 +507,7 @@ shortest_paths <- function(graph, from, to = V(graph),
                            output = c("vpath", "epath", "both"),
                            predecessors = FALSE, inbound.edges = FALSE,
                            algorithm = c("automatic", "unweighted", "dijkstra", "bellman-ford")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
     "out" = 1,
@@ -613,9 +599,7 @@ all_shortest_paths <- function(graph, from,
                                to = V(graph),
                                mode = c("out", "all", "in"),
                                weights = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
     "out" = 1,
@@ -686,9 +670,7 @@ all_shortest_paths <- function(graph, from,
 #' subcomponent(g, 1, "out")
 #' subcomponent(g, 1, "all")
 subcomponent <- function(graph, v, mode = c("all", "out", "in")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
     "out" = 1,
@@ -759,9 +741,7 @@ subgraph <- function(graph, vids) {
 #' @export
 induced_subgraph <- function(graph, vids, impl = c("auto", "copy_and_delete", "create_from_scratch")) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   vids <- as.igraph.vs(graph, vids)
   impl <- switch(igraph.match.arg(impl),
     "auto" = 0,
@@ -784,9 +764,7 @@ induced_subgraph <- function(graph, vids, impl = c("auto", "copy_and_delete", "c
 #' @export
 subgraph.edges <- function(graph, eids, delete.vertices = TRUE) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   eids <- as.igraph.es(graph, eids)
   delete.vertices <- as.logical(delete.vertices)
 
@@ -902,9 +880,7 @@ transitivity <- function(graph, type = c(
                            "barrat", "weighted"
                          ),
                          vids = NULL, weights = NULL, isolates = c("NaN", "zero")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   type <- igraph.match.arg(type)
   type <- switch(type,
     "undirected" = 0,
@@ -1012,9 +988,7 @@ transitivity <- function(graph, type = c(
 #' constraint(g)
 #'
 constraint <- function(graph, nodes = V(graph), weights = NULL) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   nodes <- as.igraph.vs(graph, nodes)
 
   if (is.null(weights)) {
@@ -1069,9 +1043,7 @@ constraint <- function(graph, nodes = V(graph), weights = NULL) {
 #'
 reciprocity <- function(graph, ignore.loops = TRUE,
                         mode = c("default", "ratio")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- switch(igraph.match.arg(mode),
     "default" = 0,
     "ratio" = 1
@@ -1124,9 +1096,7 @@ reciprocity <- function(graph, ignore.loops = TRUE,
 #' edge_density(simplify(g), loops = FALSE) # this is also right, but different
 #'
 edge_density <- function(graph, loops = FALSE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   on.exit(.Call(R_igraph_finalizer))
   .Call(R_igraph_density, graph, as.logical(loops))
@@ -1137,9 +1107,7 @@ edge_density <- function(graph, loops = FALSE) {
 #' @export
 ego_size <- function(graph, order = 1, nodes = V(graph),
                      mode = c("all", "out", "in"), mindist = 0) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
     "out" = 1,
@@ -1228,9 +1196,7 @@ ego_size <- function(graph, order = 1, nodes = V(graph),
 #'
 ego <- function(graph, order = 1, nodes = V(graph),
                 mode = c("all", "out", "in"), mindist = 0) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
     "out" = 1,
@@ -1259,9 +1225,7 @@ ego <- function(graph, order = 1, nodes = V(graph),
 #' @export
 make_ego_graph <- function(graph, order = 1, nodes = V(graph),
                            mode = c("all", "out", "in"), mindist = 0) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
     "out" = 1,
@@ -1318,9 +1282,7 @@ make_ego_graph <- function(graph, order = 1, nodes = V(graph),
 #' coreness(g) # small core triangle in a ring
 #'
 coreness <- function(graph, mode = c("all", "out", "in")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
     "out" = 1,
@@ -1369,9 +1331,7 @@ coreness <- function(graph, mode = c("all", "out", "in")) {
 #' topo_sort(g)
 #'
 topo_sort <- function(graph, mode = c("out", "all", "in")) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- igraph.match.arg(mode)
   mode <- switch(mode,
     "out" = 1,
@@ -1469,9 +1429,8 @@ feedback_arc_set <- feedback_arc_set_impl
 #' girth(g)
 #'
 girth <- function(graph, circle = TRUE) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   on.exit(.Call(R_igraph_finalizer))
   res <- .Call(R_igraph_girth, graph, as.logical(circle))
   if (igraph_opt("return.vs.es") && circle) {
@@ -1657,9 +1616,7 @@ bfs <- function(graph, root, mode = c("out", "in", "all", "total"),
                 pred = FALSE, succ = FALSE, dist = FALSE,
                 callback = NULL, extra = NULL, rho = parent.frame(),
                 neimode) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   if (!missing(neimode)) {
     warning("Argument `neimode' is deprecated; use `mode' instead")
@@ -1822,10 +1779,7 @@ dfs <- function(graph, root, mode = c("out", "in", "all", "total"),
                 order = TRUE, order.out = FALSE, father = FALSE, dist = FALSE,
                 in.callback = NULL, out.callback = NULL, extra = NULL,
                 rho = parent.frame(), neimode) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
-
+  ensure_igraph(graph)
   if (!missing(neimode)) {
     warning("Argument `neimode' is deprecated; use `mode' instead")
     if (missing(mode)) {
@@ -1929,9 +1883,7 @@ dfs <- function(graph, root, mode = c("out", "in", "all", "total"),
 #'
 components <- function(graph, mode = c("weak", "strong")) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- switch(igraph.match.arg(mode),
     "weak" = 1,
     "strong" = 2
@@ -1992,9 +1944,7 @@ count_components <- count_components
 #'
 unfold_tree <- function(graph, mode = c("all", "out", "in", "total"), roots) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   mode <- switch(igraph.match.arg(mode),
     "out" = 1,
     "in" = 2,
@@ -2054,9 +2004,7 @@ unfold_tree <- function(graph, mode = c("all", "out", "in", "total"), roots) {
 laplacian_matrix <- function(graph, normalized = FALSE, weights = NULL,
                              sparse = igraph_opt("sparsematrices")) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   normalized <- as.logical(normalized)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
@@ -2172,9 +2120,7 @@ laplacian_matrix <- function(graph, normalized = FALSE, weights = NULL,
 #' @export
 is_matching <- function(graph, matching, types = NULL) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   types <- handle_vertex_type_arg(types, graph, required = F)
   matching <- as.igraph.vs(graph, matching, na.ok = TRUE) - 1
   matching[is.na(matching)] <- -1
@@ -2191,9 +2137,7 @@ is_matching <- function(graph, matching, types = NULL) {
 #' @rdname matching
 is_max_matching <- function(graph, matching, types = NULL) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   types <- handle_vertex_type_arg(types, graph, required = F)
   matching <- as.igraph.vs(graph, matching, na.ok = TRUE) - 1
   matching[is.na(matching)] <- -1
@@ -2211,9 +2155,7 @@ is_max_matching <- function(graph, matching, types = NULL) {
 max_bipartite_match <- function(graph, types = NULL, weights = NULL,
                                 eps = .Machine$double.eps) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   types <- handle_vertex_type_arg(types, graph)
   if (is.null(weights) && "weight" %in% edge_attr_names(graph)) {
     weights <- E(graph)$weight
diff --git a/R/structure.info.R b/R/structure.info.R
index 6a77fe3f64e..6ca6d4e0d27 100644
--- a/R/structure.info.R
+++ b/R/structure.info.R
@@ -46,9 +46,8 @@
 #' are_adjacent(ug, 1, 2)
 #' are_adjacent(ug, 2, 1)
 are_adjacent <- function(graph, v1, v2) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
+
   on.exit(.Call(R_igraph_finalizer))
   .Call(
     R_igraph_are_connected, graph, as.igraph.vs(graph, v1) - 1,
diff --git a/R/tkplot.R b/R/tkplot.R
index 31bc949fe3a..162451630ce 100644
--- a/R/tkplot.R
+++ b/R/tkplot.R
@@ -167,9 +167,7 @@ assign(".next", 1, .tkplot.env)
 #' }
 #'
 tkplot <- function(graph, canvas.width = 450, canvas.height = 450, ...) {
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
 
   # Libraries
   requireNamespace("tcltk", quietly = TRUE) ||
diff --git a/R/topology.R b/R/topology.R
index 922c0e7194f..3110182dd2a 100644
--- a/R/topology.R
+++ b/R/topology.R
@@ -25,12 +25,8 @@ graph.get.isomorphisms.vf2 <- function(graph1, graph2, vertex.color1,
                                        vertex.color2, edge.color1,
                                        edge.color2) {
   # Argument checks
-  if (!is_igraph(graph1)) {
-    stop("Not a graph object")
-  }
-  if (!is_igraph(graph2)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph1)
+  ensure_igraph(graph2)
   if (missing(vertex.color1)) {
     if ("color" %in% vertex_attr_names(graph1)) {
       vertex.color1 <- V(graph1)$color
@@ -87,12 +83,8 @@ graph.get.subisomorphisms.vf2 <- function(graph1, graph2, vertex.color1,
                                           vertex.color2, edge.color1,
                                           edge.color2) {
   # Argument checks
-  if (!is_igraph(graph1)) {
-    stop("Not a graph object")
-  }
-  if (!is_igraph(graph2)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph1)
+  ensure_igraph(graph2)
   if (missing(vertex.color1)) {
     if ("color" %in% vertex_attr_names(graph1)) {
       vertex.color1 <- V(graph1)$color
@@ -147,9 +139,7 @@ graph.get.subisomorphisms.vf2 <- function(graph1, graph2, vertex.color1,
 #' @export
 graph.isoclass.subgraph <- function(graph, vids) {
   # Argument checks
-  if (!is_igraph(graph)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph)
   vids <- as.igraph.vs(graph, vids) - 1
 
   on.exit(.Call(R_igraph_finalizer))
@@ -163,12 +153,8 @@ graph.subisomorphic.lad <- function(pattern, target, domains = NULL,
                                     induced = FALSE, map = TRUE, all.maps = FALSE,
                                     time.limit = Inf) {
   # Argument checks
-  if (!is_igraph(pattern)) {
-    stop("Not a graph object")
-  }
-  if (!is_igraph(target)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(pattern)
+  ensure_igraph(target)
   induced <- as.logical(induced)
   if (time.limit == Inf) {
     time.limit <- 0L
@@ -315,12 +301,8 @@ isomorphic <- function(graph1, graph2, method = c(
                          "auto", "direct",
                          "vf2", "bliss"
                        ), ...) {
-  if (!is_igraph(graph1)) {
-    stop("Not a graph object")
-  }
-  if (!is_igraph(graph2)) {
-    stop("Not a graph object")
-  }
+  ensure_igraph(graph1)
+  ensure_igraph(graph2)
   method <- igraph.match.arg(method)
 
   if (method == "auto") {
diff --git a/R/utils-ensure.R b/R/utils-ensure.R
new file mode 100644
index 00000000000..6c66fb30487
--- /dev/null
+++ b/R/utils-ensure.R
@@ -0,0 +1,18 @@
+ensure_igraph <- function(graph, optional = FALSE) {
+
+  if (is.null(graph)) {
+    if (!optional) {
+      cli::cli_abort("Must provide a graph object (provided {.code NULL}).")
+    } else {
+      return()
+    }
+  }
+
+  if (rlang::is_missing(graph)) {
+    cli::cli_abort("Must provide a graph object (missing argument).")
+  }
+
+  if (!is_igraph(graph)) {
+    cli::cli_abort("Must provide a graph object (provided wrong object type).")
+  }
+}
diff --git a/tests/testthat/_snaps/utils-ensure.md b/tests/testthat/_snaps/utils-ensure.md
new file mode 100644
index 00000000000..69b6ac09d8c
--- /dev/null
+++ b/tests/testthat/_snaps/utils-ensure.md
@@ -0,0 +1,12 @@
+# ensure_igraph() works
+
+    Must provide a graph object (provided wrong object type).
+
+---
+
+    Must provide a graph object (provided wrong object type).
+
+---
+
+    Must provide a graph object (provided `NULL`).
+
diff --git a/tests/testthat/test-constraint.R b/tests/testthat/test-constraint.R
index df2551470c3..4408033b15c 100644
--- a/tests/testthat/test-constraint.R
+++ b/tests/testthat/test-constraint.R
@@ -1,8 +1,6 @@
 test_that("constraint works", {
   constraint.orig <- function(graph, nodes = V(graph), attr = NULL) {
-    if (!is_igraph(graph)) {
-      stop("Not a graph object")
-    }
+    ensure_igraph(graph)
     idx <- degree(graph) != 0
     A <- as_adj(graph, attr = attr, sparse = FALSE)
     A <- A[idx, idx]
diff --git a/tests/testthat/test-utils-ensure.R b/tests/testthat/test-utils-ensure.R
new file mode 100644
index 00000000000..82f00f5648f
--- /dev/null
+++ b/tests/testthat/test-utils-ensure.R
@@ -0,0 +1,7 @@
+test_that("ensure_igraph() works", {
+  expect_snapshot_error(ensure_igraph(1))
+  expect_snapshot_error(ensure_igraph(NA))
+  expect_snapshot_error(ensure_igraph(NULL))
+  expect_silent(ensure_igraph(make_empty_graph()))
+  expect_silent(ensure_igraph(NULL, optional = TRUE))
+})
diff --git a/tools/stimulus/types-RR.yaml b/tools/stimulus/types-RR.yaml
index f56dc0047c6..13d8ecf4a9c 100644
--- a/tools/stimulus/types-RR.yaml
+++ b/tools/stimulus/types-RR.yaml
@@ -1,9 +1,9 @@
 
 GRAPH:
-    INCONV: if (!is_igraph(%I%)) { stop("Not a graph object") }
+    INCONV: ensure_igraph(%I%)
 
 GRAPH_OR_0:
-    INCONV: if (!is.null(graph) && !is_igraph(%I%)) { stop("Not a graph object") }
+    INCONV: ensure_igraph(%I%, optional = TRUE)
 
 INTEGER:
     DEFAULT: