ERRNO cGenericDevice::SendCommand(unsigned _Direction, unsigned _Command, ...)
{
    m_Command.clear();

    if (SendENQ())
    {
         return ERR_NAK;
    }

    if (ReceiveACK())
    {
         return ERR_NAK;
    }

    unsigned CommandSize;
    unsigned checksum = 0xff;

    m_Command.push_back(STX);

    m_Command.push_back(_Direction);
    CommandSize++;
    checksum -= _Direction;

    m_Command.push_back(_Command);
    CommandSize++;
    checksum -= _Command;

    va_list morebytes;
    va_start(morebytes, _Command);
    unsigned char tempbyte;

    for (;;)
    {
        tempbyte = va_arg(morebytes, unsigned char);

        if (tempbyte == '\0')
        {
            break;
        }

        m_Command.push_back(static_cast<unsigned>(tempbyte));
        CommandSize++;
        checksum -= tempbyte;
    }

    va_end(morebytes);

    if (SendByte(CommandSize))
    {
        return ERR_NAK;
    }

    checksum -= CommandSize;

    m_Command.push_back(checksum);
    m_Command.push_back(ETX);

    if (SendByteVector(m_Command))
    {
        return ERR_NAK;
    }

    return ERR_OK;
}

Ich verwende g++-4.3. Das Programm bricht bei va_start mit einem SIGILL ab.
Ich kann in meinem Code aber keinen Fehler entdecken.
Was mach ich falsch?