Your request has unearthed an error in my code. I am sorry about this. To help me fix this error, please report it and the request that produced it (models, user/min/max, features) to michael@truedelta.com. I would truly appreciate this, as I cannot personally test all possible requests. Feel free to try a slightly different request--such errors are very rare. The original request should be possible within 24 hours of a report. Thank you.