ActionMailer bug?

actionmailer_bug.rb

# When everything work
class TestMailer < ActionMailer::Base
  default :from => "from@example.com", :to => "to@example.com"

  def hello
    mail do |format|
      format.text { render :text => "Hello!" }
      format.html { render :text => "<h1>Hello!</h1>" }
    end
  end

end

hello_email = TestMailer.hello
hello_email.text_part.body.to_s   # => "Hello!"
hello_email.html_part.body.to_s   # => "<h1>Hello!</h1>"


##################################################################################


# When nothing works!!!
class TestMailer < ActionMailer::Base
  default :from => "from@example.com", :to => "to@example.com"

  def hello
    mail do |format|
      format.text { render :text => "Hello!" }
      # We just commented it out
      # format.html { render :text => "<h1>Hello!</h1>" }
    end
  end

end

hello_email = TestMailer.hello
hello_email.html_part   # => nil, it's OK
hello_email.text_part   # => nil, WTF?