def add_to_watched(self, watched):
        """
        :calls: `PUT /repos/:owner/:repo/subscription <http://developer.github.com/v3/activity/watching>`_
        :param watched: :class:`github.Repository.Repository`
        :rtype: None
        """
        assert isinstance(watched, github.Repository.Repository), watched
        headers, data = self._requester.requestJsonAndCheck(
            "PUT",
            "/repos/" + watched._identity + "/subscription",
            input={"subscribed": True}
        )