diff --git a/wp-includes/nav-menu-template.php b/wp-includes/nav-menu-template.php
index 6db82b1e8f..a346f27df5 100644
--- a/wp-includes/nav-menu-template.php
+++ b/wp-includes/nav-menu-template.php
@@ -95,7 +95,7 @@ class Walker_Nav_Menu extends Walker {
 		 * @param object $item    The current menu item.
 		 * @param array  $args    An array of wp_nav_menu() arguments.
 		 */
-		$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
+		$class_names = join( ' ', (array) apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
 		$class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';
 
 		/**
diff --git a/wp-includes/version.php b/wp-includes/version.php
index c6d52ea62e..2fde33dd19 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.1-alpha-30031';
+$wp_version = '4.1-alpha-30032';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.