diff --git a/src/backend/catalog/heap.c b/src/backend/catalog/heap.c
index 2f6a6ff..e38cb86 100644
--- a/src/backend/catalog/heap.c
+++ b/src/backend/catalog/heap.c
@@ -2307,12 +2307,17 @@ MergeWithExistingConstraint(Relation rel, char *ccname, Node *expr,
 						(errcode(ERRCODE_DUPLICATE_OBJECT),
 				errmsg("constraint \"%s\" for relation \"%s\" already exists",
 					   ccname, RelationGetRelationName(rel))));
-			/* OK to update the tuple */
-			ereport(NOTICE,
-			   (errmsg("merging constraint \"%s\" with inherited definition",
-					   ccname)));
+
 			tup = heap_copytuple(tup);
 			con = (Form_pg_constraint) GETSTRUCT(tup);
+
+			/* If the constraint is "only" then cannot merge */
+			if (con->conisonly)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+				errmsg("constraint \"%s\" for relation \"%s\" is an ONLY constraint. Cannot merge",
+					   ccname, RelationGetRelationName(rel))));
+
 			if (is_local)
 				con->conislocal = true;
 			else
@@ -2322,6 +2327,10 @@ MergeWithExistingConstraint(Relation rel, char *ccname, Node *expr,
 				Assert(is_local);
 				con->conisonly = true;
 			}
+			/* OK to update the tuple */
+			ereport(NOTICE,
+			   (errmsg("merging constraint \"%s\" with inherited definition",
+					   ccname)));
 			simple_heap_update(conDesc, &tup->t_self, tup);
 			CatalogUpdateIndexes(conDesc, tup);
 			break;