diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index 7d0279ec3ef..b441f7e5cac 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -697,9 +697,13 @@ generate_nonunion_path(SetOperationStmt *op, PlannerInfo *root,
 	/* Identify the grouping semantics */
 	groupList = generate_setop_grouplist(op, tlist);
 
-	/* punt if nothing to group on (can this happen?) */
+	/* punt if nothing to group on (not worth fixing in back branches) */
 	if (groupList == NIL)
-		return path;
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 /* translator: %s is UNION, INTERSECT, or EXCEPT */
+				 errmsg("%s over no columns is not supported",
+						(op->op == SETOP_INTERSECT) ? "INTERSECT" : "EXCEPT")));
 
 	/*
 	 * Estimate number of distinct groups that we'll need hashtable entries
@@ -850,9 +854,12 @@ make_union_unique(SetOperationStmt *op, Path *path, List *tlist,
 	/* Identify the grouping semantics */
 	groupList = generate_setop_grouplist(op, tlist);
 
-	/* punt if nothing to group on (can this happen?) */
+	/* punt if nothing to group on (not worth fixing in back branches) */
 	if (groupList == NIL)
-		return path;
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 /* translator: %s is UNION, INTERSECT, or EXCEPT */
+				 errmsg("%s over no columns is not supported", "UNION")));
 
 	/*
 	 * XXX for the moment, take the number of distinct groups as equal to the