problems while updating gem

# gem update --system Updating RubyGems... Attempting remote update of rubygems-update Successfully installed rubygems-update-1.0.0 Updating version of RubyGems to 1.2.0 ERROR: While executing gem ... (Errno::ENOENT)     No such file or directory - /usr/lib/ruby/gems/1.8/gems/rubygems- update-1.2.0

does anybody know, why it's loading rubygems-update-1.0.0 but looking for rubygems-update-1.2.0? what can i do?