diff -u b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
--- b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
+++ b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
@@ -632,7 +632,7 @@
    * @param string[] $contexts
    *   A set of cache contexts used for the regular (pre-bubbling) CID.
    *
-   * @return null|string
+   * @return string|null
    *   The redirected (post-bubbling) CID, if any.
    */
   protected function createRedirectedCacheId(array $keys, array $contexts) {