NoMethodError: `bytesize' for 200:Fixnum (with Rails-3.0.3)

Thanks to Rails developer team.

I tried Rails3 along this guide:

Ruby on Rails Guides: Getting Started with Rails
http://edgeguides.rubyonrails.org/getting_started.html

and I met the problem described as following.
...Any ideas?

(tentative solution)

This problem seems to be related to rack(1.2.1).
I added this code to config/environment.rb, so tentatively, it works.

require "rack/content_length"
class << (Rack::ContentLength)
  def new(app)
    app
  end
end