It's not exactly a bug, but it's an artifact of the rough interface between DJ and DemeterJ, which is still very new and has some kinks to be worked out. Yep, your diagnosis is correct. However, DJ has no way of knowing that this field is not meant to be part of the class graph.
Any help is appreciated.
Feel free to send us mail as soon as you get stuck, so if it turns out to be a problem with the Demeter software then you don't have to spin your wheels. But it can't hurt to ask.