def do_execute(self):
        """
        The actual execution of the actor.

        :return: None if successful, otherwise error message
        :rtype: str
        """
        conv = self.config["setup"].shallow_copy()
        conv.input = self._input.payload
        result = conv.convert()
        if result is None:
            if conv.output is not None:
                self._output.append(Token(conv.output))
        return None