diff --git a/core/lib/Drupal/Core/File/FileUrlGenerator.php b/core/lib/Drupal/Core/File/FileUrlGenerator.php
index 10e4f38a6f..3eabb8b4a2 100644
--- a/core/lib/Drupal/Core/File/FileUrlGenerator.php
+++ b/core/lib/Drupal/Core/File/FileUrlGenerator.php
@@ -79,7 +79,7 @@ public function generateAbsoluteString($uri) {
    *   file. If a stream wrapper could not be found to generate an external URL,
    *   then FALSE is returned.
    */
-  protected function doGenerateString(string $uri, bool $relative) {
+  protected function doGenerateString($uri, $relative) {
     // Allow the URI to be altered, e.g. to serve a file from a CDN or static
     // file server.
     $this->moduleHandler->alter('file_url', $uri);