custom post types error

function my_post_types($types) {
    $types[] = 'portfolio';
    return $types;
}

add_filter('s2_post_types', 'portfolio');

I’m getting this error after putting the above code into my functions.php:

Subscribe2 will send email notifications for the following custom post types:
Warning: Invalid argument supplied for foreach() in /home/lovealot/public_html/wp-content/plugins/subscribe2/admin/settings.php on line 222

https://ww.wp.xz.cn/plugins/subscribe2/