RuntimeError: Set changed size during iteration

[bakwc]

Exception, cron build failed: https://travis-ci.org/github/bakwc/PySyncObj/jobs/690781983

        self.setOnReadonlyNodeDisconnectedCallback(None)

        for node in self._nodes | self._readonlyNodes:

            self.dropNode(node)

        if self._server is not None:

            self._server.unbind()

>       for conn in self._unknownConnections:

E       RuntimeError: Set changed size during iteration