Internal flash with extension tubes causing short.

I've discovered that one of my auto extension tubes causes the camera to lockup if I depress the shutter with the internal flash open. The screen just goes black but thankfully it can be cleared by removing and replacing the battery. It doesn't happen with the other two. They are the type where I had to scrape away a spot of the paint to provide the additional contact. I suppose it is contact related but they all look the same. This is the only situation I have noticed this happen and with a flash on the hotshoe everything is normal. It's not a big deal really now I know but wondered if anyone else has had similar or has any ideas.
Dave