diff --git a/src/map.c b/src/map.c
index eed3a55..6a994d4 100644
--- a/src/map.c
+++ b/src/map.c
@@ -2304,6 +2304,8 @@
     char_u	*arg;
 
     which = tv_get_string_buf_chk(&argvars[0], buf);
+    if (which == NULL)
+	return;
     mode = get_map_mode(&which, 0);
     is_abbr = (int)tv_get_number(&argvars[1]);