This is BUG that was reported and fixed a while ago. Do you have V18.2, Build 73.1 installed? When Line Width Units is set "mm" and Format was set to "Architectural" in the "Units in Workspace" section of "Options / Space Units", I would get this error.
DoubleCAD is also infected with this BUG, but they seem to deny that anything is wrong.
Designer, Deluxe, (Professional, Expert, Basic), Platinum
Windows 10 Pro (1803), 64-bit