Playlist request sort problem

Hi There

When I try to sort a playlist by artist_start_year-desc, this doesn't work. For example it will come back with Foo Fighters->Placebo->Rod Stewart->Van Morrison->Placebo.

I could run the sort myself however it seems that it's not possible to get the artist start year from a playlist request.

This is my url:

http://developer.echonest.com/api/v4/playlist/static?api_key=XXX&type=catalog&genre=rock&results=10&bucket=audio_summary&seed_catalog=CAUJAWN14B4642B412&sort=artist_start_year-desc