Rebel Xsi and old aftermarket lens

I just purchased a Rebel Xsi and one of the big reasons was I already had a couple of canon lenses from an EOS 620 film camera. I also have a Sigma 400mm tele. When I try to use the tele at anything less than about 30 feet I get an error message that says:
Err 01
Communication between the lens and the camera is faulty. Clean the contacts.

Seems that anything past about 30 feet is fine. Any ideas..
(I know, just use it past 30 feet )

Thanks,
Tom (the new guy)