Custom Taxonomy show up as "invalid" when I try to add an item to Menu

Hi everyone.

It’s been awhile since I’ve been working with WordPress, but is the first time I try to create taxonomies and post types.

This is my code

function portfolio_register() {

    $labels = array(
        'name' => _x('Portfolio', 'post type general name'),
        'singular_name' => _x('Portfolio Item', 'post type singular name'),
        'add_new' => _x('Add New', 'portfolio item'),
        'add_new_item' => __('Add New Portfolio Item'),
        'edit_item' => __('Edit Portfolio Item'),
        'new_item' => __('New Portfolio Item'),
        'view_item' => __('View Portfolio Item'),
        'search_items' => __('Search Portfolio'),
        'not_found' => __('Nothing found'),
        'not_found_in_trash' => __('Nothing found in Trash'),
        'parent_item_colon' => ''
    );

    $args = array(
        'labels' => $labels,
        'public' => true,
        'publicly_queryable' => true,
        'show_ui' => true,
        'query_var' => true,
        'menu_icon' => get_stylesheet_directory_uri() . '/images/project.png',
        'rewrite' => true,
        'capability_type' => 'post',
        'hierarchical' => false,
        'supports' => array('title', 'editor', 'thumbnail')
    );

    register_post_type('portfolio', $args);

    register_taxonomy("types", "portfolio", array(
        "hierarchical" => true,
        "label" => "Types",
        "labels" => array(
            "name" => "Types",
            "singular_label" => "Type"))
    );
}

add_action('init', 'portfolio_register');

All works fine, except when I try to add an Type item to the menu. The box that appears in the menu, is red, and has a label saying “Invalid”. What am I doing wrong?

Thanks