This works when I try it in MRI. But this is what happens in Macruby. Is there a workaround? $ macirb -f irb(main):003:0> require 'aws/s3' => true irb(main):006:0> AWS::S3::Base.establish_connection!( irb(main):007:0> :access_key_id => ACCESS_KEY, irb(main):008:0> :secret_access_key => SECRET_ACCESS_KEY irb(main):009:0> ) => #<AWS::S3::Connection:0x40043ed60 @options={:server=>" s3.amazonaws.com", :port=>80, :access_key_id=>"XXXX", :secret_access_key=>"XXXX"} @access_key_id="XXXX" @secret_access_key="XXXX" @http=#<Net::HTTP s3.amazonaws.com:80 open=false>> irb(main):010:0> AWS::S3::Service.buckets NameError: `@policy:' is not allowed as an instance variable name /Users/jeremysmith/.rvm/gems/macruby-0.10/gems/aws-s3-0.6.2/lib/aws/s3/extensions.rb:142:in `expirable_memoize:' /Users/jeremysmith/.rvm/gems/macruby-0.10/gems/aws-s3-0.6.2/lib/aws/s3/extensions.rb:176:in `policy:'