diff --git a/core/lib/Drupal/Core/Routing/UrlGenerator.php b/core/lib/Drupal/Core/Routing/UrlGenerator.php
index d9d8e48c2b..b758071921 100644
--- a/core/lib/Drupal/Core/Routing/UrlGenerator.php
+++ b/core/lib/Drupal/Core/Routing/UrlGenerator.php
@@ -439,7 +439,7 @@ protected function getRoute($name) {
    *
    * @internal
    */
-  private function getRouteStringIdentifier($name) {
+  private function getRouteStringIdentifier($name): string {
     if (is_scalar($name)) {
       return $name;
     }