Identify the error in this Ruby class using respond_to_missing? and method_missing:

Identify the error in this Ruby class using respond_to_missing? and method_missing:

class Broken
  def respond_to_missing?(method_name, include_private = false)
    method_name.to_s.start_with?("do_")
  end

  def method_missing(method_name, *args)
    "Doing #{method_name}"
  end
end

b = Broken.new
puts b.do_something
puts b.unknown