diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index f864af5d667..c94799b15c7 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -672,10 +672,13 @@ _copyMergeJoin(const MergeJoin *from)
 	 */
 	COPY_NODE_FIELD(mergeclauses);
 	numCols = list_length(from->mergeclauses);
-	COPY_POINTER_FIELD(mergeFamilies, numCols * sizeof(Oid));
-	COPY_POINTER_FIELD(mergeCollations, numCols * sizeof(Oid));
-	COPY_POINTER_FIELD(mergeStrategies, numCols * sizeof(int));
-	COPY_POINTER_FIELD(mergeNullsFirst, numCols * sizeof(bool));
+	if (numCols > 0)
+	{
+		COPY_POINTER_FIELD(mergeFamilies, numCols * sizeof(Oid));
+		COPY_POINTER_FIELD(mergeCollations, numCols * sizeof(Oid));
+		COPY_POINTER_FIELD(mergeStrategies, numCols * sizeof(int));
+		COPY_POINTER_FIELD(mergeNullsFirst, numCols * sizeof(bool));
+	}
 
 	return newnode;
 }