diff --git a/src/vim9execute.c b/src/vim9execute.c
index c2a7de5..455a205 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -4234,7 +4234,8 @@
 	    semsg(_(e_nr_arguments_too_many), idx);
 	goto failed_early;
     }
-    else if (idx < 0)
+    idx = argc - ufunc->uf_args.ga_len + ufunc->uf_def_args.ga_len;
+    if (idx < 0)
     {
 	if (idx == -1)
 	    emsg(_(e_one_argument_too_few));