Wordpress - add div wrapper around sub-menu

sub-menu-wrap

/* EXTEND SUBNAV
******************************************/

class submenu-wrap extends Walker_Nav_Menu {
    function start_lvl( &$output, $depth = 0, $args = array() ) {
        $indent = str_repeat("\t", $depth);
        $output .= "\n$indent<div class='sub-menu-wrap'><ul class='sub-menu'>\n";
    }
    function end_lvl( &$output, $depth = 0, $args = array() ) {
        $indent = str_repeat("\t", $depth);
        $output .= "$indent</ul></div>\n";
    }
}

<?php wp_nav_menu(array(
	'walker' => new submenu-wrap()
)); ?>