diff --git a/core/modules/book/src/BookManager.php b/core/modules/book/src/BookManager.php
index 06dd84fdc..c4c0396b5 100644
--- a/core/modules/book/src/BookManager.php
+++ b/core/modules/book/src/BookManager.php
@@ -211,6 +211,10 @@ protected function findChildrenRelativeDepth(array $book_link) {
    * {@inheritdoc}
    */
   public function addFormElements(array $form, FormStateInterface $form_state, NodeInterface $node, AccountInterface $account, $collapsed = TRUE) {
+    if (!$node->book) {
+      return $form;
+    }
+
     // If the form is being processed during the Ajax callback of our book bid
     // dropdown, then $form_state will hold the value that was selected.
     if ($form_state->hasValue('book')) {