diff --git a/core/tests/Drupal/KernelTests/Core/Extension/ExtensionUnserializedServiceAppRootUsageTest.php b/core/tests/Drupal/KernelTests/Core/Extension/ExtensionUnserializedServiceAppRootUsageTest.php
index ec214d125f..f32463e687 100644
--- a/core/tests/Drupal/KernelTests/Core/Extension/ExtensionUnserializedServiceAppRootUsageTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Extension/ExtensionUnserializedServiceAppRootUsageTest.php
@@ -29,6 +29,7 @@ class ExtensionUnserializedServiceAppRootUsageTest extends KernelTestBase {
    */
   protected function setUp() {
     parent::setUp();
+    // Instantiate an Extension object for testing unserialization.
     $this->extension = new Extension($this->container->get('app.root'), 'module', 'core/modules/system/system.info.yml', 'system.module');
     // Set a dummy container app.root for testing.
     $this->dummyAppRoot = '/dummy/app/root';
@@ -42,13 +43,13 @@ protected function setUp() {
    * object data, test that the Extension object's root property is set to
    * the container's app.root and not the DRUPAL_ROOT constant, if the service
    * container app.root is available.
+   *
+   * @covers ::unserialize
    */
   public function testServiceAppRouteUsage() {
     $data = $this->extension->serialize();
     $this->extension->unserialize($data);
-    $reflection_root = new \ReflectionProperty($this->extension, 'root');
-    $reflection_root->setAccessible(true);
-    self::assertEquals($this->dummyAppRoot, $reflection_root->getValue($this->extension));
+    self::assertEquals($this->dummyAppRoot, $this->readAttribute($this->extension, 'root'));
   }
 
 }