diff --git a/core/modules/book/lib/Drupal/book/BookManager.php b/core/modules/book/lib/Drupal/book/BookManager.php
index 857c5b5..1593c23 100644
--- a/core/modules/book/lib/Drupal/book/BookManager.php
+++ b/core/modules/book/lib/Drupal/book/BookManager.php
@@ -481,7 +481,7 @@ protected function recurseTableOfContents($tree, $indent, &$toc, $exclude, $dept
       if (!in_array($data['link']['mlid'], $exclude)) {
         $toc[$data['link']['mlid']] = $indent . ' ' . truncate_utf8($data['link']['title'], 30, TRUE, TRUE);
         if ($data['below']) {
-          recurseTableOfContents($data['below'], $indent . '--', $toc, $exclude, $depth_limit);
+          $this->recurseTableOfContents($data['below'], $indent . '--', $toc, $exclude, $depth_limit);
         }
       }
     }