No moods found

Query 1, song/search -> mood=sad

http://developer.echonest.com/api/v4/song/search?api_key=XX&format=json&mood=sad

Partial response:

"songs": [{"artist_id": "ARUCN341187B9A7308", "id": "SOEGMUC131F71D65A0", "artist_name": "Boys Suck", "title": "The Chicken Cow"}, 
{"artist_id": "AR0DS7Q1187FB53B01", "id": "SOROXDE12A8C13DCBD", "artist_name": "Damien Rice & David Gray", "title": "Que Sera, Sera - Live From Live Earth"}

Query 2, artist/terms -> type=mood, first ID from query 1

http://developer.echonest.com/api/v4/artist/terms?api_key=XX&format=json&id=ARUCN341187B9A7308&type=mood

Response:

"terms": [{"frequency": 0.61077496555870858, "name": "pop punk", "weight": 0.97619047619047616}, 
{"frequency": 0.7869333523105454, "name": "indie", "weight": 0.76190476190476186}, 
{"frequency": 1.0, "name": "rock", "weight": 0.59523809523809523}]}}

I don't see 'sad' in there, anywhere

Query 3, artist/terms -> type=mood, second ID from query 1

http://developer.echonest.com/api/v4/artist/terms?api_key=XX&format=json&id=AR0DS7Q1187FB53B01&type=mood

Response:

"terms": []}

So apparently this artist can be found with mood 'sad' but the reverse isn't true, can anyone explain?