inheritable_copy error

mailer.rb

class MyMailer < ActionMailer::Base

  def notice(user)
    # ...
    mail(to: @to, from: @from, subject: @subject) do |format|
      format.html { render text: @html_content }
      format.text { render text: @text_content }
    end
  end
end

# WARN: undefined method `inheritable_copy' for #<Rails::Application::Configuration:0xb134c90>

# traced to:
# https://github.com/rails/rails/blob/3-2-stable/actionpack/lib/action_view/helpers/controller_helper.rb#L16