diff --git a/src/eval.c b/src/eval.c
index fd028cc..7776cc6 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -9386,7 +9386,8 @@
 	char_u	buf[NUMBUFLEN];
 	char	*error = (char *)get_tv_string_buf_chk(&argvars[1], buf);
 
-	if (strstr((char *)vimvars[VV_ERRMSG].vv_str, error) == NULL)
+	if (error == NULL
+		  || strstr((char *)vimvars[VV_ERRMSG].vv_str, error) == NULL)
 	{
 	    prepare_assert_error(&ga);
 	    fill_assert_error(&ga, &argvars[2], NULL, &argvars[1],