Index: class-walker-nav-menu.php
===================================================================
--- class-walker-nav-menu.php	(revision 38038)
+++ class-walker-nav-menu.php	(working copy)
@@ -85,6 +85,7 @@
 	 * @param int    $id     Current item ID.
 	 */
 	public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
+		$args = (array) $args; // cast to array prevents object-by-reference side effects
 		$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
 
 		$classes = empty( $item->classes ) ? array() : (array) $item->classes;
@@ -99,7 +100,7 @@
 		 * @param object $item  Menu item data object.
 		 * @param int    $depth Depth of menu item. Used for padding.
 		 */
-		$args = apply_filters( 'nav_menu_item_args', $args, $item, $depth );
+		$args = apply_filters( 'nav_menu_item_args', (object) $args, $item, $depth );
 
 		/**
 		 * Filters the CSS class(es) applied to a menu item's list item element.