diff --git a/matemat/webserver/test/test_post.py b/matemat/webserver/test/test_post.py
index 059d41b..ad99247 100644
--- a/matemat/webserver/test/test_post.py
+++ b/matemat/webserver/test/test_post.py
@@ -17,7 +17,7 @@ def post_test_pagelet(method: str,
     headers['Content-Type'] = 'text/plain'
     dump: str = ''
     for k, v in args.items():
-        dump += f'{k}: {v if v is str else ",".join(v)}\n'
+        dump += f'{k}: {v if isinstance(v, str) else ",".join(v)}\n'
     return 200, dump