diff --git a/R/data.table.R b/R/data.table.R
index 7d3acef2fa..f225855a3c 100644
--- a/R/data.table.R
+++ b/R/data.table.R
@@ -756,7 +756,14 @@ replace_dot_alias = function(e) {
           bysub = parse(text=paste0("list(",paste(bysub,collapse=","),")"))[[1L]]
           bysubl = as.list.default(bysub)
         }
-        allbyvars = intersect(all.vars(bysub), names_x)
+        # Fix 4981: when the 'by' expression includes get/mget/eval, all.vars
+        # cannot be trusted to infer all used columns
+        bysub.elems <- rapply(as.list(bysub), as.character)
+        if (any(c("eval","evalq","eval.parent","local","get","mget","dynGet") %chin% bysub.elems)) 
+          allbyvars = NULL
+        else
+          allbyvars = intersect(all.vars(bysub), names_x)  
+        
         orderedirows = .Call(CisOrderedSubset, irows, nrow(x))  # TRUE when irows is NULL (i.e. no i clause). Similar but better than is.sorted(f__)
         bysameorder = byindex = FALSE
         if (!bysub %iscall% ":" && ##Fix #4285
diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index 819c1ba0ca..2610c22724 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -17515,3 +17515,14 @@ test(2183.79, melt(DTid, measure.vars=measure(letter, number, pattern=as.integer
 test(2183.80, melt(DTid, measure.vars=measure(letter, number, sep=as.integer)), error="sep must be character string")
 ##melt(DTid, measure.vars=measure(letter, number, sep=NA_character_)
 ##melt(DTid, measure.vars=measure(letter, number, sep=character())
+
+# `keyby` allows mixing eval/get with direct columns, #4981
+dt <- data.table(a=c(1,2), b=c(3,4), c=c(1,0))
+dt2 <- dt[,.(suma=sum(a)), keyby=.(b=get("b"),c)]
+test(2184.1, dt2[1, suma], 1)
+dt2 <- dt[2,.(suma=sum(a)), keyby=.(b=b,c)]
+test(2184.2, dt2[1, suma], 2)
+dt2 <- dt[2,.(suma=sum(a)), keyby=.(b=get("b"))]
+test(2184.3, dt2[1, suma], 2)
+dt2 <- dt[2,.(suma=sum(a)), keyby=.(b=get("b"),c)]
+test(2184.4, dt2[1, suma], 2)
\ No newline at end of file