Ruby on Rails

Trying to understand why I get this error. I tried to follow
instructions at :
http://www.hackido.com/2010/12/install-ruby-on-rails-on-ubuntu.html

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Here is some more info on the issue:
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root@ubuntu-VirtualBox:/home/adminuser/Desktop/rubygems-1.4.0# rvmsudo
gem install rails --no-rdoc --no-ri
ERROR: Loading command: install (LoadError)
no such file to load -- zlib
ERROR: While executing gem ... (NameError)
uninitialized constant Gem::Commands::InstallCommand
root@ubuntu-VirtualBox:/home/adminuser/Desktop/rubygems-1.4.0# gem list
ERROR: Loading command: list (LoadError)
no such file to load -- zlib
ERROR: While executing gem ... (NameError)
uninitialized constant Gem::Commands::ListCommand
root@ubuntu-VirtualBox:/home/adminuser/Desktop/rubygems-1.4.0# which gem
/usr/local/rvm/rubies/ruby-1.9.2-p136/bin/gem

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