diff --git a/wp-includes/post-template.php b/wp-includes/post-template.php
index 4aa48c74e..c3bfa2bb4 100644
--- a/wp-includes/post-template.php
+++ b/wp-includes/post-template.php
@@ -1020,6 +1020,7 @@ class Walker_Page extends Walker {
 		$css_class = array('page_item', 'page-item-'.$page->ID);
 		if ( !empty($current_page) ) {
 			$_current_page = get_page( $current_page );
+			_get_post_ancestors($_current_page);
 			if ( isset($_current_page->ancestors) && in_array($page->ID, (array) $_current_page->ancestors) )
 				$css_class[] = 'current_page_ancestor';
 			if ( $page->ID == $current_page )