Couldn't download youtube captions

[james cui]

Hey guys,

I was trying the Youtube Data API today and wanted to download captions of some videos, but couldn't make it work. Here is what I did, I picked a popular video and listed all captions, and then I picked the caption id that translated to English. It throws at YouTube.Captions.download call no matter what format parameter I use. Here is the detailed error "GoogleJsonResponseException: API call to youtube.captions.download failed with error: Unsupported Output Format" Did anyone encounter this issue? Any advice to resolve this?

function myFunction() {
  var ret = YouTube.Captions.list(['id,snippet'], 'M7FIvfx5J10');
  console.log(ret);
 
  var obj = new Object();
  obj['tlang'] = 'en';
  obj['tfmt'] = 'ttml';
  //sbv – SubViewer subtitle
  //scc – Scenarist Closed Caption format
  //srt – SubRip subtitle
  //ttml – Timed Text Markup Language caption
  //vtt – Web Video Text Tracks caption
  var ret1 = YouTube.Captions.download('8yMV7mc691aTqjNxZ9zPHOdwpkL_e11j', obj);

  console.log(ret1);
}