diff --git a/src/eval.c b/src/eval.c
index 0d83f1d..34fdd67 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -9278,7 +9278,8 @@
 	assert_error(&ga);
 	ga_clear(&ga);
     }
-    else if (strstr((char *)vimvars[VV_EXCEPTION].vv_str, error) == NULL)
+    else if (error != NULL
+	    && strstr((char *)vimvars[VV_EXCEPTION].vv_str, error) == NULL)
     {
 	prepare_assert_error(&ga);
 	fill_assert_error(&ga, &argvars[1], NULL, &argvars[0],