diff --git a/Tests/Fixtures/MockStream/MockStream.php b/Tests/Fixtures/MockStream/MockStream.php
index cb8ed6a77..bf4c1466c 100644
--- a/Tests/Fixtures/MockStream/MockStream.php
+++ b/Tests/Fixtures/MockStream/MockStream.php
@@ -28,7 +28,7 @@ class MockStream
      * @param string|null $opened_path If the path is opened successfully, and STREAM_USE_PATH is set in options,
      *                                 opened_path should be set to the full path of the file/resource that was actually opened
      */
-    public function stream_open(string $path, string $mode, int $options, string &$opened_path = null): bool
+    public function stream_open(string $path, string $mode, int $options, ?string &$opened_path = null): bool
     {
         return true;
     }