QSslCertificate: serial number is -1 instead of real serial number

i have written a small app which checks an ssl enabled servers certificate. When I ouput the serial number (lets say www.google.com) it prints out -1.

QSslConfiguration conf = socket->sslConfiguration();
this->peercert = conf.peerCertificate();
qDebug() << this->peercert.serialNumber();

If i debug the certificate there is a correct one (of course):
peercert->d->x509->cert_info->serialNumber

Maybe there is a bug in qsslcertificate.cpp, function serialNumber():

QByteArray QSslCertificate::serialNumber() const
{
    // maybe serialNumberString is NOT empty and gets never updated
    if (d->serialNumberString.isEmpty() && d->x509)
        d->serialNumberString =
            QByteArray::number(qlonglong(q_ASN1_INTEGER_get(d->x509->cert_info->serialNumber)));

    return d->serialNumberString;
}