Index: wp-includes/class-wp-walker.php
===================================================================
--- wp-includes/class-wp-walker.php	(revision 23338)
+++ wp-includes/class-wp-walker.php	(working copy)
@@ -123,7 +123,7 @@
 		$id_field = $this->db_fields['id'];
 
 		//display this element
-		if ( is_array( $args[0] ) )
+		if ( isset( $args[0] ) && is_array( $args[0] ) )
 			$args[0]['has_children'] = ! empty( $children_elements[$element->$id_field] );
 		$cb_args = array_merge( array(&$output, $element, $depth), $args);
 		call_user_func_array(array($this, 'start_el'), $cb_args);