diff --git src/wp-admin/includes/ajax-actions.php src/wp-admin/includes/ajax-actions.php
index cf1e92b..d96a2fb 100644
--- src/wp-admin/includes/ajax-actions.php
+++ src/wp-admin/includes/ajax-actions.php
@@ -1196,7 +1196,7 @@ function wp_ajax_add_menu_item() {
 	/** This filter is documented in wp-admin/includes/nav-menu.php */
 	$walker_class_name = apply_filters( 'wp_edit_nav_menu_walker', 'Walker_Nav_Menu_Edit', $_POST['menu'] );
 
-	if ( ! class_exists( $walker_class_name, false ) )
+	if ( ! class_exists( $walker_class_name ) )
 		wp_die( 0 );
 
 	if ( ! empty( $menu_items ) ) {
diff --git src/wp-admin/includes/nav-menu.php src/wp-admin/includes/nav-menu.php
index 5944f93..16b157e 100644
--- src/wp-admin/includes/nav-menu.php
+++ src/wp-admin/includes/nav-menu.php
@@ -899,7 +899,7 @@ function wp_get_nav_menu_to_edit( $menu_id = 0 ) {
 		 */
 		$walker_class_name = apply_filters( 'wp_edit_nav_menu_walker', 'Walker_Nav_Menu_Edit', $menu_id );
 
-		if ( class_exists( $walker_class_name, false ) ) {
+		if ( class_exists( $walker_class_name ) ) {
 			$walker = new $walker_class_name;
 		} else {
 			return new WP_Error( 'menu_walker_not_exist',
diff --git src/wp-includes/admin-bar.php src/wp-includes/admin-bar.php
index 162e680..2bf56a8 100644
--- src/wp-includes/admin-bar.php
+++ src/wp-includes/admin-bar.php
@@ -39,7 +39,7 @@ function _wp_admin_bar_init() {
 	 * @param string $wp_admin_bar_class Admin bar class to use. Default 'WP_Admin_Bar'.
 	 */
 	$admin_bar_class = apply_filters( 'wp_admin_bar_class', 'WP_Admin_Bar' );
-	if ( class_exists( $admin_bar_class, false ) )
+	if ( class_exists( $admin_bar_class ) )
 		$wp_admin_bar = new $admin_bar_class;
 	else
 		return false;