Extend walker class with custom post types

I’m using the walker class below, but I can’t make it work with custom posts.

 class Zwp_Walker_Nav_Menu_Dynamic extends Walker_Nav_Menu {
    function end_el(&$output, $item, $depth) {
        global $wp_query;
        global $post;
        $thePostID = $post->ID;

        $subposts = get_posts( 'numberposts=10&cat=' . $item->object_id );

        if ( $subposts ) {

            $output .= '        <ul class="';

            if ( $item->object_id == $post->ID && 100 == 101) { $output .= ' sub-menu-current'; } else { $output .= ' sub-menu'; }
            $output .=  ' sub-menu-item-'.$item->object_id.'">
            ';
            foreach( $subposts as $post ) :
                setup_postdata( $post );
                $output .= '<li';
                if ( is_single() && $wp_query->post->ID == $post->ID ) { 
                    $output .= ' class="current"'; 
                }
                $output .= '><a href="'. get_permalink( $post->ID ) .'">' . get_the_title( $post->ID ) . '</a></li>
                ';
            endforeach;
            $output .= '</ul>
            ';

        }

    }
}   

I tried to change the get_posts query, but I get strange results. (Probably because the ID is missing)

$subposts = get_posts( 'numberposts=10&post_type=portfolio_work' );

The result:

Design »
    Project 1
    Project 2
Photography »
    Project 1
    Project 2
About »
    Project 1
    Project 2

This is how it should look like:

Design »
    Project 1
Photography »
    Project 2
About