WordPress export tool doesn’t export post-format value of custom post type?

After spending time debugging export.php, I found that we have to register post_format taxonomy for our custom post type.

register_taxonomy( 'post_format', 'portfolio', array(
            'public' => true,
            'hierarchical' => false,
            'labels' => array(
                    'name' => _x( 'Format', 'post format' ),
                    'singular_name' => _x( 'Format', 'post format' ),
            ),
            'query_var' => true,
            'rewrite' => $rewrite['post_format'],
            'show_ui' => false,
            '_builtin' => true,
            'show_in_nav_menus' => current_theme_supports( 'post-formats' ),
    ) );

Easy as that!