diff --git a/core/modules/system/src/Tests/Update/StableBaseThemeUpdateTest.php b/core/modules/system/src/Tests/Update/StableBaseThemeUpdateTest.php
index 565fa0d..338e767 100644
--- a/core/modules/system/src/Tests/Update/StableBaseThemeUpdateTest.php
+++ b/core/modules/system/src/Tests/Update/StableBaseThemeUpdateTest.php
@@ -40,6 +40,7 @@ protected function setUp() {
    * Tests that the Stable base theme is installed if necessary.
    */
   public function testUpdateHookN() {
+    /** @var \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler */
     $theme_handler = \Drupal::service('theme_handler');
     $this->assertTrue($theme_handler->themeExists('test_stable'));
     $this->assertFalse($theme_handler->themeExists('stable'));