Index: src/wp-includes/class-walker-nav-menu.php
===================================================================
--- src/wp-includes/class-walker-nav-menu.php	(revision 7eb3daf5626b6596110e8d7ed70ecdfaf3d27383)
+++ src/wp-includes/class-walker-nav-menu.php	(date 1524076443474)
@@ -116,6 +116,7 @@
 	 * @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
 		if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
 			$t = '';
 			$n = '';
@@ -137,7 +138,7 @@
 		 * @param WP_Post  $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 classes applied to a menu item's list item element.