test using os.path.sep not hardcoded /
diff --git a/certbot/tests/cert_manager_test.py b/certbot/tests/cert_manager_test.py
index bffa5298fcb..e04e25da8b6 100644
--- a/certbot/tests/cert_manager_test.py
+++ b/certbot/tests/cert_manager_test.py
@@ -383,7 +383,7 @@ def test_rename_cert_bad_new_certname(self, mock_check, unused_get_utility):
         mock_config.new_certname = "example.org"
         self.assertRaises(errors.ConfigurationError, self._call, mock_config)
 
-        mock_config.new_certname = "one/two"
+        mock_config.new_certname = "one{0}two".format(os.path.sep)
         self.assertRaises(errors.ConfigurationError, self._call, mock_config)
 
 
