diff --git a/routemaster/config/tests/test_loading.py b/routemaster/config/tests/test_loading.py
index 8af92ac1..ab9e2ef7 100644
--- a/routemaster/config/tests/test_loading.py
+++ b/routemaster/config/tests/test_loading.py
@@ -309,10 +309,10 @@ def test_environment_variables_override_config_file_for_database_config():
 
 def test_raises_for_unparseable_database_port_in_environment_variable():
     with mock.patch.dict(os.environ, {'DB_PORT': 'not an int'}):
-        with assert_config_error(f"Could not parse DB_PORT as an integer: 'not an int'."):
+        with assert_config_error("Could not parse DB_PORT as an integer: 'not an int'."):
             load_config(yaml_data('realistic'))
 
 
 def test_multiple_feeds_same_name_invalid():
-    with assert_config_error(f"Feeds must have unique names at state_machines.example.feeds"):
+    with assert_config_error("Feeds must have unique names at state_machines.example.feeds"):
         load_config(yaml_data('multiple_feeds_same_name_invalid'))