def getblockhash(self, height):
        """Return hash of block in best-block-chain at height.

        Raises IndexError if height is not valid.
        """
        try:
            return lx(self._call('getblockhash', height))
        except JSONRPCException as ex:
            raise IndexError('%s.getblockhash(): %s (%d)' %
                    (self.__class__.__name__, ex.error['message'], ex.error['code']))