ruby 1.9.3 gem install rails 出错

ruby 1.9.3 gem install rails 报错
错误：

$gem install rails
rdoc --op /usr/local/lib/ruby/gems/1.9.1/doc/rails-3.2.5/rdoc lib --title rails-3.2.5 Documentation --quiet
file 'lib' not found

解决方法：

$gem install rails --no-ri --no-rdoc