diff -u b/core/modules/system/tests/src/FunctionalJavascript/ModuleFilterTest.php b/core/modules/system/tests/src/FunctionalJavascript/ModuleFilterTest.php
--- b/core/modules/system/tests/src/FunctionalJavascript/ModuleFilterTest.php
+++ b/core/modules/system/tests/src/FunctionalJavascript/ModuleFilterTest.php
@@ -24,7 +24,7 @@
   /**
    * {@inheritdoc}
    */
-  protected function setUp() {
+  protected function setUp(): void {
     parent::setUp();
 
     $admin_user = $this->drupalCreateUser([