I am wanting to dispaly custom post types on my archive pages using code from here:

https://wordpress.stackexchange.com/questions/179023/adding-custom-post-types-to-archive-php

but this now gives WP error message of:

Warning: Invalid argument supplied for foreach() in /wp-includes/class-wp-post-type.php on line 613

add_action('pre_get_posts', 'query_post_type');

function query_post_type($query) {
  if($query->is_main_query()
    && ( is_category() || is_tag() )) {
        $query->set( 'post_type', array('post','cpt') );
  }
}

Please let me know what changes are needed to not return errors.