diff --git a/src/backend/utils/adt/dbsize.c b/src/backend/utils/adt/dbsize.c
index 3c70bb5943..144aca1099 100644
--- a/src/backend/utils/adt/dbsize.c
+++ b/src/backend/utils/adt/dbsize.c
@@ -905,6 +905,9 @@ pg_filenode_relation(PG_FUNCTION_ARGS)
 	Oid			relfilenode = PG_GETARG_OID(1);
 	Oid			heaprel = InvalidOid;
 
+	if (!OidIsValid(relfilenode))
+		PG_RETURN_NULL();
+
 	heaprel = RelidByRelfilenode(reltablespace, relfilenode);
 
 	if (!OidIsValid(heaprel))