diff --git a/conservancy_beancount/config.py b/conservancy_beancount/config.py
index fd9f27869595f67d036aa698af342c7e9ab74d41..95d763e5461cc66effb3febee5e5a803d8672b0d 100644
--- a/conservancy_beancount/config.py
+++ b/conservancy_beancount/config.py
@@ -65,6 +65,18 @@ class RTCredentials(NamedTuple):
         else:
             return cls(**values)
 
+    def idstr(self) -> str:
+        """Return a string unique to these credentials
+
+        This returns a string that incorporates the server URL and user.
+        The string will be unique across different credentials.
+        It's suitable for use as a cache key or filename.
+        """
+        return '{}@{}'.format(
+            self.user or '',
+            urlparse.quote(self.server or '', ''),
+        )
+
 
 class Config:
     _ENVIRON_DEFAULT_PATHS = {
@@ -209,11 +221,7 @@ class Config:
         if cache_dir_path is None:
             cache_db = None
         else:
-            cache_name = '{}@{}.sqlite3'.format(
-                credentials.user,
-                urlparse.quote(str(credentials.server), ''),
-            )
-            cache_path = cache_dir_path / cache_name
+            cache_path = cache_dir_path / f'{credentials.idstr()}.sqlite3'
             try:
                 cache_path.touch(0o600)
             except OSError:
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)