def issuer(self, value):
        """
        An asn1crypto.x509.Certificate or oscrypto.asymmetric.Certificate object
        of the issuer.
        """

        is_oscrypto = isinstance(value, asymmetric.Certificate)
        if not is_oscrypto and not isinstance(value, x509.Certificate):
            raise TypeError(_pretty_message(
                '''
                issuer must be an instance of asn1crypto.x509.Certificate or
                oscrypto.asymmetric.Certificate, not %s
                ''',
                _type_name(value)
            ))

        if is_oscrypto:
            value = value.asn1

        self._issuer = value