diff -u b/core/tests/Drupal/Tests/Component/Serialization/YamlSymfonyTest.php b/core/tests/Drupal/Tests/Component/Serialization/YamlSymfonyTest.php
--- b/core/tests/Drupal/Tests/Component/Serialization/YamlSymfonyTest.php
+++ b/core/tests/Drupal/Tests/Component/Serialization/YamlSymfonyTest.php
@@ -10,6 +10,7 @@
  * @group Drupal
  * @group Serialization
  * @coversDefaultClass \Drupal\Component\Serialization\YamlSymfony
+ * @requires extension yaml
  */
 class YamlSymfonyTest extends YamlTestBase {