It's a non-cpu error for the lens. If the lens *has* a chip then the contacts could probably do with a good clean.
From the net somewhere...
"To fix that problem with 50mm f1.8 lens, detach the lens and set the aperture to f22, lock the lens aperture by pushing the red switch downwards, attache the lens to the camera body(don't forget to hear the lens click else u will get f-- error) and finally unlock the lens aperture and it works normally. enjoy clicking"