Haze - Exclude Portfolio Featured Image from Slider

$args = array(
    'post_type' => 'attachment',
    'orderby' => 'menu_order',
    'post_status' => null,
    'post_parent' => get_the_ID(),
    'post_mime_type' => 'image',
    'exclude'     => get_post_thumbnail_id(),
    'posts_per_page' => -1
);