diff --git a/src/data.table.h b/src/data.table.h
index f4d22b95a..70f5a9bb1 100644
--- a/src/data.table.h
+++ b/src/data.table.h
@@ -286,6 +286,7 @@ SEXP notchin(SEXP x, SEXP table);
 
 // mergelist.c
 SEXP cbindlist(SEXP x, SEXP copyArg);
+SEXP copyCols(SEXP x, SEXP cols);
 
 // functions called from R level .Call/.External and registered in init.c
 // these now live here to pass -Wstrict-prototypes, #5477
diff --git a/src/init.c b/src/init.c
index 1ee424314..02959bdcd 100644
--- a/src/init.c
+++ b/src/init.c
@@ -151,6 +151,7 @@ R_CallMethodDef callMethods[] = {
 {"Cnotchin", (DL_FUNC)&notchin, -1},
 {"Ccbindlist", (DL_FUNC) &cbindlist, -1},
 {"CperhapsDataTableR", (DL_FUNC) &perhapsDataTableR, -1},
+{"CcopyCols", (DL_FUNC) &copyCols, -1},
 {"Cwarn_matrix_column_r", (DL_FUNC)&warn_matrix_column_r, -1},
 {NULL, NULL, 0}
 };
diff --git a/src/mergelist.c b/src/mergelist.c
index 77c428773..60c508977 100644
--- a/src/mergelist.c
+++ b/src/mergelist.c
@@ -1,5 +1,22 @@
 #include "data.table.h"
 
+// set(x, NULL, cols, copy(unclass(x)[cols])) ## but keeps the index
+SEXP copyCols(SEXP x, SEXP cols) {
+  // used in R/mergelist.R
+  if (!isDataTable(x))
+    error("'x' must be a data.table"); // # nocov
+  if (!isInteger(cols))
+    error("'cols' must be integer"); // # nocov
+  int nx = length(x), ncols = LENGTH(cols), *colsp = INTEGER(cols);
+  if (!nx || !ncols)
+    return R_NilValue;
+  for (int i=0; i<ncols; ++i) {
+    int thiscol = colsp[i]-1;
+    SET_VECTOR_ELT(x, thiscol, duplicate(VECTOR_ELT(x, thiscol)));
+  }
+  return R_NilValue;
+}
+
 void mergeIndexAttrib(SEXP to, SEXP from) {
   if (!isInteger(to) || LENGTH(to)!=0)
     internal_error(__func__, "'to' must be integer() already"); // # nocov