Incorrect error: missing item id or song_id/artist_id

I'm getting this error when trying to insert items into a tasteprofile:

message: 'Invalid item block in the input: missing item id or song_id/artist_id: {"action": "update", "item": {"item_id": 528, "artist_id": "ARLIJWL14372A40AFB"}}' } } }

The error seems wrong, you can clearly see both "item_id" and "artist_id" being returned back with the error message, so I know the server got the JSON and properly parsed it. I've tried several permutations of omitting one or the other fields, or using just "artist_name". I've also tried using the exact body of one of the examples in the API documentation: { "action": "update""item": { "item_id": "itsgucci", "play_count": 10 } }

All to no avail. Same frustrating error message.

Any ideas?