diff --git a/src/utils.c b/src/utils.c
index 20f6c6d5c..34da9ddc1 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -502,6 +502,7 @@ static inline bool equalLens(SEXP x) {
   return true;
 }
 
+// setDT()-friendly rectangular list, i.e.
 // is.list(x) && equalLens(x) && (!length(x) || !is.null(names(x)))
 bool isDataList(SEXP x) {
   return isNewList(x) && (!LENGTH(x) || (equalLens(x) && !isNull(getAttrib(x, R_NamesSymbol))));