diff --git a/core/modules/book/book.admin.inc b/core/modules/book/book.admin.inc
index d112180..59fa298 100644
--- a/core/modules/book/book.admin.inc
+++ b/core/modules/book/book.admin.inc
@@ -92,8 +92,10 @@ function book_admin_settings_validate($form, &$form_state) {
  * @see book_admin_settings_validate()
  */
 function book_admin_settings_submit($form, &$form_state) {
+  // Save allowed types as indexed values (sample, 0 => book).
+  $allowed_types = array_values(array_filter($form_state['values']['book_allowed_types']));
   config('book.settings')
-    ->set('allowed_types', $form_state['values']['book_allowed_types'])
+    ->set('allowed_types', $allowed_types)
     ->set('child_type', $form_state['values']['book_child_type'])
     ->save();
  }