diff --git a/tests/lib/SimpleSAML/Store/RedisTest.php b/tests/lib/SimpleSAML/Store/RedisTest.php
index 44ab5215e6d58c1c63c97c3fa47ee08537c3e530..46811c186743f76940a198be1e4b68eb82db8a6e 100644
--- a/tests/lib/SimpleSAML/Store/RedisTest.php
+++ b/tests/lib/SimpleSAML/Store/RedisTest.php
@@ -73,9 +73,9 @@ 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;
     }