diff --git a/src/eval.c b/src/eval.c
index 1516fe5..0784215 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -9580,7 +9580,9 @@
     char_u	*pat = get_tv_string_buf_chk(&argvars[0], buf1);
     char_u	*text = get_tv_string_buf_chk(&argvars[1], buf2);
 
-    if (!pattern_match(pat, text, FALSE))
+    if (pat == NULL || text == NULL)
+	EMSG(_(e_invarg));
+    else if (!pattern_match(pat, text, FALSE))
     {
 	prepare_assert_error(&ga);
 	fill_assert_error(&ga, &argvars[2], NULL, &argvars[0], &argvars[1],