Wordpress pre_get_posts钩子在Wordpress“页面”帖子类型上不起作用？

I have a Wordpress "page" (post type), with slug "/mypage". On the page (using template page-mypage.php) I'm trying to show a custom list of posts, so I thought I'd tap into the "pre_get_posts" filter to do that. Inside the template page-mypage.php I do something like the following to try and filter the output of the Main Loop:

function getuserfavs($query){
  if ($query->is_main_query()){
        $query->set('post_type', 'post');
        $query->set('post__in', array(130, 128));
  }
}

add_action('pre_get_posts', 'getuserfavs');

That doesn't work, however, and all I get when viewing "/mypage" is a 404 page not found template. The Main loop if I look into it isn't altered either. Can anyone explain why this is so? Am I missing something in terms of how 'pre_get_posts' should be used?

Thanks,