Problem with "gem cleanup" on OS X 10.5

When I try to execute 'sudo gem cleanup' on OS X 10.5, I get the  
message:

Attempting to uninstall rails-1.2.6
ERROR: While executing gem . . . (Gem:InstallError)
     Unknown gem rails = 1.2.6

How can I fix this so gem cleanup will continue to completion?

Thanks,
Ken