c4ux

adding this to functions.php worked for me:

add_filter( 'instant_articles_post_types', 'add_post_types', 10,1 );
function add_post_types($post_type_array){
    array_push($post_type_array,'post');
    array_push($post_type_array,'customposttype');
	return $post_type_array;
}