diff --git a/tests/lib/SimpleSAML/Store/RedisTest.php b/tests/lib/SimpleSAML/Store/RedisTest.php
index 915934493c82414d82317a3163b45b4c1497eb49..92b6dc53c0475dc8a07bc7a484d23ff516498db2 100644
--- a/tests/lib/SimpleSAML/Store/RedisTest.php
+++ b/tests/lib/SimpleSAML/Store/RedisTest.php
@@ -68,9 +68,9 @@ class RedisTest extends TestCase
 
     /**
      * @param string $key
-     * @return \Predis\Client
+     * @return \Predis\Client|null
      */
-    public function getMocked($key)
+    public function getMocked(string $key)
     {
         return array_key_exists($key, $this->config) ? $this->config[$key] : null;
     }