diff --git a/NEWS.md b/NEWS.md
index 295b231b80..3c57114fd8 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -9,6 +9,8 @@
 
 2. `keyby=` on columns for which an index exists now uses the index (new feature 7 in v1.11.6 below) but if an `i` subset is present in the same query then it could segfault, [#3062](https://github.com/Rdatatable/data.table/issues/3062). Again thanks to @renkun-ken for reporting.
 
+3. Assigning an out-of-range integer to an item in a factor column (a rare operation) correctly created an `NA` in that spot with warning, but now no longer also corrupts the variable being assigned, [#2984](https://github.com/Rdatatable/data.table/issues/2984). Thanks to @radfordneal for reporting and @MarkusBonsch for fixing. Assigning a string which is missing from the factor levels continues to automatically append the string to the factor levels.
+
 
 ### Changes in v1.11.6  (on CRAN 19 Sep 2018)
 
diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index 264b4be07e..497b7ddbed 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -1218,7 +1218,7 @@ DT[1,a:="A"]
 test(428, DT, data.table(a=factor(c("A","Z","A","Z")),b=1:4))
 DT[1,a:=2L]
 test(429, DT, data.table(a=factor(c("Z","Z","A","Z")),b=1:4))
-test(430, DT[1,1]<- 3L, NA_integer_, warning="RHS contains 3 which is outside the levels range.*1,2.*of column 1, NAs generated")
+test(430, DT[1,1]<- 3L, 3L, warning="RHS contains 3 which is outside the levels range.*1,2.*of column 1, NAs generated")
 test(431, DT[1,1:=4L], data.table(a=factor(c(NA,"Z","A","Z")),b=1:4), warning="RHS contains 4 which is outside the levels range.*1,2.*of column 1, NAs generated")
 
 
@@ -12239,6 +12239,13 @@ test(1944.4, DT[flag == 1 & group == 1, x], 6L)
 test(1944.5, indices(DT), "group__flag")
 test(1944.6, DT[flag == 1, sum(x), keyby = group], data.table(group=1:4, V1=INT(6,3,18,17), key="group"))
 
+## tests for issue #2984
+DT <- data.table(a = factor(c("A", "Z")), b = 1:4)
+c <- 3L
+DT[1, a:=c]
+test(1945.1, c, 3L)
+DT[2,1]<- c 
+test(1945.2, c, 3L)
 
 ###################################
 #  Add new tests above this line  #
diff --git a/src/assign.c b/src/assign.c
index c7dadbe243..c1e943d82c 100644
--- a/src/assign.c
+++ b/src/assign.c
@@ -575,7 +575,11 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values, SEXP v
             protecti++;
             // silence warning on singleton NAs
             if (INTEGER(RHS)[0] != NA_INTEGER) warning("Coerced '%s' RHS to 'integer' to match the factor column's underlying type. Character columns are now recommended (can be in keys), or coerce RHS to integer or character first.", type2char(TYPEOF(thisvalue)));
-          } else RHS = thisvalue;
+          } else {
+            // make sure to copy thisvalue. May be modified below. See #2984
+            RHS = PROTECT(duplicate(thisvalue));
+            protecti++;
+          }
           for (j=0; j<length(RHS); j++) {
             if ( (INTEGER(RHS)[j]<1 || INTEGER(RHS)[j]>LENGTH(targetlevels))
                  && INTEGER(RHS)[j] != NA_INTEGER) {