diff --git a/core/modules/page_cache/src/Tests/PageCacheTest.php b/core/modules/page_cache/src/Tests/PageCacheTest.php
index b3b0430..761210a 100644
--- a/core/modules/page_cache/src/Tests/PageCacheTest.php
+++ b/core/modules/page_cache/src/Tests/PageCacheTest.php
@@ -308,7 +308,8 @@ public function testPageCacheAnonymousRolePermissions() {
    */
   function testPageCacheAnonymous403404() {
     $admin_url = Url::fromRoute('system.admin');
-    $invalid_url = 'foo/' . $this->randomString();
+    // Use a random string without any illegal characters.
+    $invalid_url = 'foo/' . $this->randomMachineName();
     $tests = [
       403 => $admin_url,
       404 => $invalid_url,