diff --git a/core/lib/Drupal/Core/File/FileSystem.php b/core/lib/Drupal/Core/File/FileSystem.php
index f01b0515b0..7d4bca5fca 100644
--- a/core/lib/Drupal/Core/File/FileSystem.php
+++ b/core/lib/Drupal/Core/File/FileSystem.php
@@ -545,7 +545,7 @@ public function saveData($data, $destination = NULL, $replace = self::FILE_EXIST
   /**
    * {@inheritdoc}
    */
-  public function prepareDirectory($directory, $options = FileSystemInterface::FILE_MODIFY_PERMISSIONS) {
+  public function prepareDirectory(&$directory, $options = FileSystemInterface::FILE_MODIFY_PERMISSIONS) {
     if (!$this->validScheme($this->uriScheme($directory))) {
       // Only trim if we're not dealing with a stream.
       $directory = rtrim($directory, '/\\');
diff --git a/core/lib/Drupal/Core/File/FileSystemInterface.php b/core/lib/Drupal/Core/File/FileSystemInterface.php
index 03d0109392..e0ef0f52d5 100644
--- a/core/lib/Drupal/Core/File/FileSystemInterface.php
+++ b/core/lib/Drupal/Core/File/FileSystemInterface.php
@@ -423,6 +423,6 @@ public function saveData($data, $destination = NULL, $replace = self::FILE_EXIST
    *   TRUE if the directory exists (or was created) and is writable. FALSE
    *   otherwise.
    */
-  public function prepareDirectory($directory, $options = FileSystemInterface::FILE_MODIFY_PERMISSIONS);
+  public function prepareDirectory(&$directory, $options = FileSystemInterface::FILE_MODIFY_PERMISSIONS);
 
 }