diff --git a/tests/test_config.py b/tests/test_config.py
index efa2ad872be028db7fab83cb8b4ad4ab66f30c66..d124ede8ac5457a7ae5b097d3c45200fc016dece 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -7,6 +7,7 @@
 
 import contextlib
 import decimal
+import itertools
 import operator
 import os
 import re
@@ -140,6 +141,17 @@ def test_rt_credentials_from_all_sources_mixed(tmp_path):
         rt_credentials = config.rt_credentials()
     assert rt_credentials == (server, 'mixedup', 'mixed up', 'rt')
 
+def test_rt_credentials_idstr():
+    actual = {
+        config_mod.RTCredentials(server, user).idstr()
+        for server, user in itertools.product(
+                [None, 'https://example.org/rt'],
+                [None, 'example'],
+        )}
+    assert len(actual) == 4
+    for idstr in actual:
+        assert '/' not in idstr
+
 def check_rt_client_url(credentials, client):
     pattern = '^{}/?$'.format(re.escape(credentials[0].rstrip('/') + '/REST/1.0'))
     assert re.match(pattern, client.url)