diff --git a/core/lib/Drupal/Core/StringTranslation/StringTranslationTrait.php b/core/lib/Drupal/Core/StringTranslation/StringTranslationTrait.php
index 7a53c68..5b46054 100644
--- a/core/lib/Drupal/Core/StringTranslation/StringTranslationTrait.php
+++ b/core/lib/Drupal/Core/StringTranslation/StringTranslationTrait.php
@@ -46,9 +46,13 @@ protected function getTranslationManager() {
    *
    * @param \Drupal\Core\StringTranslation\TranslationInterface $translation
    *   The string translation service.
+   *
+   * @return $this
    */
   public function setTranslationManager(TranslationInterface $translation) {
     $this->translationManager = $translation;
+
+    return $this;
   }
 
 }
diff --git a/core/modules/book/lib/Drupal/book/BookManager.php b/core/modules/book/lib/Drupal/book/BookManager.php
index 33ff7af..ac2ef16 100644
--- a/core/modules/book/lib/Drupal/book/BookManager.php
+++ b/core/modules/book/lib/Drupal/book/BookManager.php
@@ -12,6 +12,7 @@
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Language\Language;
 use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
 use Drupal\Core\StringTranslation\TranslationInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\node\NodeInterface;