diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/sort/SortPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/sort/SortPluginBase.php
index 587208f..e788f54 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/sort/SortPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/sort/SortPluginBase.php
@@ -179,7 +179,7 @@ public function sortSubmit(&$form, &$form_state) { }
    * Provide a list of options for the default sort form.
    * Should be overridden by classes that don't override sort_form
    */
-  function sortOptions() {
+  protected function sortOptions() {
     return array(
       'ASC' => t('Sort ascending'),
       'DESC' => t('Sort descending'),