[YouTube-API] Add a video to playlist doesn't work

Hey guys. Do you know what I'm doing wrong? $playlist->getPlaylistVideoFeedUrl() returns NULL

$feed = $yt->getVideoFeed('http://gdata.youtube.com/feeds/api/playlists/'.$input['id'].'?v=2');
$playlist = new Zend_Gdata_YouTube_PlaylistListEntry($feed->getDOM());

foreach($playlist as $video) {
  $id = $video->getVideoId();
  $video_ids[] = $id;
  if(!in_array($id,$new_videos)) {
    /* Remove the video */
  }
}

$post_url = $playlist->getPlaylistVideoFeedUrl();
foreach(array_diff($new_videos,$video_ids) as $new_video) {
  $video_entry = $yt->getVideoEntry($new_video);
  $playlist_entry = $yt->newPlaylistListEntry($video_entry->getDOM());
  $yt->insertEntry($playlist_entry,$post_url);
}

Any help is appreciated

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