diff --git a/core/lib/Drupal/Core/Entity/EntityFormController.php b/core/lib/Drupal/Core/Entity/EntityFormController.php
index 675fa98..078a8f3 100644
--- a/core/lib/Drupal/Core/Entity/EntityFormController.php
+++ b/core/lib/Drupal/Core/Entity/EntityFormController.php
@@ -64,7 +64,7 @@ public function getBaseFormID() {
     // twice otherwise.
     $base_form_id = $this->entity->entityType() . '_form';
     if ($base_form_id == $this->getFormID()) {
-      $base_form_id = '';
+      $base_form_id = NULL;
     }
     return $base_form_id;
   }
diff --git a/core/lib/Drupal/Core/Form/BaseFormIdInterface.php b/core/lib/Drupal/Core/Form/BaseFormIdInterface.php
index f948aac..3576758 100644
--- a/core/lib/Drupal/Core/Form/BaseFormIdInterface.php
+++ b/core/lib/Drupal/Core/Form/BaseFormIdInterface.php
@@ -18,8 +18,8 @@
   /**
    * Returns a string identifying the base form.
    *
-   * @return string|false
-   *   The string identifying the base form or FALSE if this is not a base form.
+   * @return string|null
+   *   The string identifying the base form or NULL if this is not a base form.
    */
   public function getBaseFormID();
 
diff --git a/core/modules/book/lib/Drupal/book/Form/BookOutlineForm.php b/core/modules/book/lib/Drupal/book/Form/BookOutlineForm.php
index d0960f4..94d9b8a 100644
--- a/core/modules/book/lib/Drupal/book/Form/BookOutlineForm.php
+++ b/core/modules/book/lib/Drupal/book/Form/BookOutlineForm.php
@@ -7,14 +7,14 @@
 
 namespace Drupal\book\Form;
 
-use Drupal\Core\Entity\EntityFormControllerNG;
+use Drupal\Core\Entity\ContentEntityFormController;
 use Drupal\book\BookManager;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
  * Displays the book outline form.
  */
-class BookOutlineForm extends EntityFormControllerNG {
+class BookOutlineForm extends ContentEntityFormController {
 
   /**
    * The book being displayed.
@@ -51,7 +51,7 @@ public static function create(ContainerInterface $container) {
    * {@inheritdoc}
    */
   public function getBaseFormID() {
-    return FALSE;
+    return NULL;
   }
 
   /**
diff --git a/core/modules/book/lib/Drupal/book/Tests/BookTest.php b/core/modules/book/lib/Drupal/book/Tests/BookTest.php
index 4772dfa..5ea8b8d 100644
--- a/core/modules/book/lib/Drupal/book/Tests/BookTest.php
+++ b/core/modules/book/lib/Drupal/book/Tests/BookTest.php
@@ -509,4 +509,17 @@ public function testBookOrdering() {
     $this->assertFieldByName("table[book-admin-{$node2->id()}][weight]", 2);
     $this->assertFieldByName("table[book-admin-{$node2->id()}][plid]", $plid);
   }
+
+  /**
+   * Tests outline of a book.
+   */
+  public function testBookOutline() {
+    // Create new book.
+    $this->drupalLogin($this->book_author);
+    $book = $this->createBookNode('new');
+
+    $this->drupalLogin($this->admin_user);
+    $this->drupalGet('node/' . $book->id() . '/outline');
+    $this->assertRaw(t('Book outline'));
+  }
 }