diff --git a/src/eval.c b/src/eval.c
index 359956b..590834f 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -12993,7 +12993,8 @@
     char_u	*pat = get_tv_string_chk(&argvars[0]);
 
     rettv->v_type = VAR_STRING;
-    rettv->vval.v_string = file_pat_to_reg_pat(pat, NULL, NULL, FALSE);
+    rettv->vval.v_string = (pat == NULL)
+			 ? NULL : file_pat_to_reg_pat(pat, NULL, NULL, FALSE);
 }
 
 /*