404 When Editing Posts

I’m using Yinjie Soon’s DF-Style Linked List plugin. Rather than modifying the theme I’m using, I’m using separate code to change the behavior of the links on my post titles:

add_filter('post_link','df_fix_permalink');

function df_fix_permalink () {
  if (get_the_linked_list_link() == '') {
    return get_permalink();
  }
  else {
    return get_the_linked_list_link();
  }
}

This works fine: for posts with a linked_list_url it changes the post title link to that URL. However, attempts to edit the post now end up in a 404. To edit a post, I must comment out add_filter().

Any suggestions to fix this?