public function addMultiSignatureAddress($nrequired, array $keys, $account = null)
    {
        // Can't use null for $account. Must be left out of the args completely when not specified.
        $args = [
            (int)$nrequired,
            $keys
        ];
        if (null !== $account) {
            $args[] = (string)$account;
        }
        return $this->rpc->query("addMultiSigAddress", $args);
    }