Praktica PLC3 meter off by two stops with Tamron adapter

I have a Praktica PLC3, an M42 mount camera, which uses an electrical connection to to the lens to allow open aperture metering. It meters correctly in open aperture mode with the Pentacon 50mm/f1.8 lens. When I use it with a Tamron 35-80 f3.8-4 lens and a Tamron electric adapter it overexposes by about a stop and a half.

As I understand it, the Praktica electric mount system expects the lens to tell it what the maximum aperture is and what it will be when the camera stops the lens down. With the Tamron zoom mounted my camera is acting as though it thinks that the maximum aperture is 1.8 instead of f3.8. Short of buying an electric lens, how can I determine whether the fault is in the camera or the adapter?