# File lib/dbus/bus.rb, line 580
    def on_return(m, &retc)
      # Have a better exception here
      if m.message_type != Message::METHOD_CALL
        raise "on_return should only get method_calls"
      end
      @method_call_msgs[m.serial] = m
      @method_call_replies[m.serial] = retc
    end