diff --git a/src/include/fmgr.h b/src/include/fmgr.h
index 0fe7b4ebc77..ed362f935f9 100644
--- a/src/include/fmgr.h
+++ b/src/include/fmgr.h
@@ -206,7 +206,7 @@ extern void fmgr_symbol(Oid functionId, char **mod, char **fn);
  * If function is not marked "proisstrict" in pg_proc, it must check for
  * null arguments using this macro.  Do not try to GETARG a null argument!
  */
-#define PG_ARGISNULL(n)  (fcinfo->args[n].isnull)
+#define PG_ARGISNULL(n)  ((n) >= fcinfo->nargs || fcinfo->args[n].isnull)
 
 /*
  * Support for fetching detoasted copies of toastable datatypes (all of