diff --git a/core/tests/Drupal/KernelTests/Core/Cache/GenericCacheBackendUnitTestBase.php b/core/tests/Drupal/KernelTests/Core/Cache/GenericCacheBackendUnitTestBase.php
index 4882824..d5f3e95 100644
--- a/core/tests/Drupal/KernelTests/Core/Cache/GenericCacheBackendUnitTestBase.php
+++ b/core/tests/Drupal/KernelTests/Core/Cache/GenericCacheBackendUnitTestBase.php
@@ -89,7 +89,7 @@ public function tearDownCacheBackend() {
    * @return \Drupal\Core\Cache\CacheBackendInterface
    *   Cache backend to test.
    */
-  protected function getCacheBackend($bin = null) {
+  protected function getCacheBackend($bin = NULL) {
     if (!isset($bin)) {
       $bin = $this->getTestBin();
     }