diff --git a/core/modules/book/lib/Drupal/book/Form/BookOutlineForm.php b/core/modules/book/lib/Drupal/book/Form/BookOutlineForm.php
index ea1f4e0..802bc2e 100644
--- a/core/modules/book/lib/Drupal/book/Form/BookOutlineForm.php
+++ b/core/modules/book/lib/Drupal/book/Form/BookOutlineForm.php
@@ -75,9 +75,7 @@ public function buildForm(array $form, array &$form_state, Node $node = NULL) {
   /**
    * {@inheritdoc}
    */
-  public function validateForm(array &$form, array &$form_state) {
-    // ...
-  }
+  public function validateForm(array &$form, array &$form_state) {}
 
   /**
    * {@inheritdoc}