diff --git a/tests/cache/tests.py b/tests/cache/tests.py
index 4bc428a98b..d18a917106 100644
--- a/tests/cache/tests.py
+++ b/tests/cache/tests.py
@@ -1256,7 +1256,7 @@ class BaseMemcachedTests(BaseCacheTests):
         Whilst other backends merely warn, memcached should raise for an
         invalid key.
         """
-        msg = expected_warning.replace(key, ':1:%s' % key)
+        msg = expected_warning.replace(key, cache.make_key(key))
         with self.assertRaisesMessage(InvalidCacheKey, msg):
             cache.set(key, 'value')