Show custom post types on tag, category and archive pages in Wordpress

gistfile1.php

<?php
/**
 * Also show custom post types in tag/category/archive pages
 */
function query_post_type($query) {
  if(is_category() || is_tag()) {
    $post_type = get_query_var('post_type');
    if($post_type) {
      $post_type = $post_type;
    } else {
      $post_type = array('nav_menu_item','post','custom_post_type','another_custom_post_type');
    }
    $query->set('post_type',$post_type);
    return $query;
  }
}
add_filter('pre_get_posts', 'query_post_type');
?>

Thanks @boyvanamstel, your function works great! I hope soon Wordpress.org could fix this error inside the core :)

Thank you. I was starting to pull my hair after an hour and a half of looking for this bug!

You rock dude!!!

You have an issue in your code that could cause a menu disappearing. You must add the 'nav_menu_item' when you do: $post_type = $post_type;. You must modify that line to this: $post_type = array( 'nav_menu_item', $post_type ); (sorry for my bad english) –