diff --git a/core/modules/book/src/BookManager.php b/core/modules/book/src/BookManager.php
index 478ff86..fba85ad 100644
--- a/core/modules/book/src/BookManager.php
+++ b/core/modules/book/src/BookManager.php
@@ -251,6 +251,8 @@ public function updateOutline(NodeInterface $node) {
 
     // Prevent changes to the book outline if the node being saved is not the
     // default revision.
+    // @todo Convert this to an entity constraint in
+    //   https://www.drupal.org/node/2883868.
     if (!$node->isDefaultRevision()) {
       drupal_set_message($this->t('This is not the default revision. You can only change the book outline for the <em>published</em> version of this content.'), 'error');
       return FALSE;