Ruby 1.9.1 gem command - can't find user for 500

< ^ > P N |<>|^_>< --- | ~ ...Help
Has anyone else encountered this error or can suggest a way to correct it? 
These errors occur when running the gem command under ruby 1.9.1.
The same error occurs with a different user #(0) when run as root.
Under 1.8.6 and 1.8.7, no error occurs.

This is a fresh version of ruby built on a 64 bit machine
OS: Linux - Fedora Core 11
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bash-/ruby-1.9.1-p243$ ruby -v
ruby 1.9.1p243 (2009-07-16 revision 24175) [i686-linux]
bash-/ruby-1.9.1-p243$ gem install rake
ERROR:  While executing gem ... (ArgumentError)
    can't find user for 500
bash-/ruby-1.9.1-p243$ gem list

*** LOCAL GEMS ***

ERROR:  While executing gem ... (ArgumentError)
    can't find user for 500
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TIA for all replies,

-- Steve Downie