Adding search bar to menu

I am trying to add a search bar to the nav_menu, by adding this to the bottom of functions.php however it will not display.

add_filter('wp_nav_menu_items','add_search', 10, 2);

function add_search($items, $args)
{

    if( $args->theme_location == 'primary' )

        return $items . '<li>'.get_search_form().'</li>';

}

Any ideas on way it seems to ignore this?