I'm going through the tekpub rack tutorial but when I try to run even a basic program in rack i get this error. 

ERROR Rack::Lint::LintError: Response body must respond to each
/Users/adam/.rvm/gems/ruby-1.9.3-preview1/gems/rack-1.3.4/lib/rack/lint.rb:19:in     `assert'
/Users/adam/.rvm/gems/ruby-1.9.3-preview1/gems/rack-1.3.4/lib/rack/lint.rb:513:in `each'
/Users/adam/.rvm/gems/ruby-1.9.3-preview1/gems/rack-1.3.4/lib/rack/body_proxy.rb:23:in `method_missing'
/Users/adam/.rvm/gems/ruby-1.9.3-preview1/gems/rack-1.3.4/lib/rack/chunked.rb:23:in `each'
/Users/adam/.rvm/gems/ruby-1.9.3-preview1/gems/rack-1.3.4/lib/rack/handler/webrick.rb:71:in `service'
/Users/adam/.rvm/rubies/ruby-1.9.3-preview1/lib/ruby/1.9.1/webrick/httpserver.rb:138:in `service'
/Users/adam/.rvm/rubies/ruby-1.9.3-preview1/lib/ruby/1.9.1/webrick/httpserver.rb:94:in `run'
/Users/adam/.rvm/rubies/ruby-1.9.3-preview1/lib/ruby/1.9.1/webrick/server.rb:191:in `block in start_thread'

This is the program im trying to run: 

class EnvironmentOutput

  def call(env)
     ["200",{"Content-Type" => "text/plain"}, "Hello World"]
  end

end

run EnvironmentOutput.new

I'm a beginner programmer so I'm not sure whats going on. Tried google searches but nothing has come up.

Using ruby 1.9.3 Rack 1.1

Thanks

ruby-on-rails ruby rack
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