I tried to start a vehicle construction using butter, but it was too slipery to ride in

DEBUG : item butter used by construction is not base item of any vehicle part!

FUNCTION : vpart_id vpart_from_item(const itype_id&)
FILE : src/construction.cpp
LINE : 1254


(I told a NPC to start vehicle construction, and was crafting 2 jam&butter sandwiches when this happened).

Edit: it asks me for vehicle name, which I enter. Then the NPC immidiately re-starts the construction. There is no vehicle there.

If I interrupt the NPC at 98% complete of this restart, it results in no vehicle.

Edit2: If I start construction myself (not by blueprint) and then have NPC continue it, the same error happens.

Edit3: and the same error happens if I start construction, have NPC do 90% and then I finish it (telling the NPC to go do something else).