diff --git a/src/backend/utils/adt/dbsize.c b/src/backend/utils/adt/dbsize.c
index 515e30d1777..ff2a0160bfb 100644
--- a/src/backend/utils/adt/dbsize.c
+++ b/src/backend/utils/adt/dbsize.c
@@ -926,7 +926,11 @@ pg_filenode_relation(PG_FUNCTION_ARGS)
 {
 	Oid			reltablespace = PG_GETARG_OID(0);
 	Oid			relfilenode = PG_GETARG_OID(1);
-	Oid			heaprel = InvalidOid;
+	Oid			heaprel;
+
+	/* test needed so RelidByRelfilenode doesn't misbehave */
+	if (!OidIsValid(relfilenode))
+		PG_RETURN_NULL();
 
 	heaprel = RelidByRelfilenode(reltablespace, relfilenode);