def company_vat(self):
        """
        Returns 10 character tax identification number,
        Polish: Numer identyfikacji podatkowej.

        https://pl.wikipedia.org/wiki/NIP
        """
        vat_digits = []

        for _ in range(3):
            vat_digits.append(self.random_digit_not_null())

        for _ in range(6):
            vat_digits.append(self.random_digit())

        check_digit = company_vat_checksum(vat_digits)

        # in this case we must generate a tax number again, because check_digit
        # cannot be 10
        if check_digit == 10:
            return self.company_vat()

        vat_digits.append(check_digit)

        return ''.join(str(digit) for digit in vat_digits)