Change boost thread priority in Windows

Im trying to change the thread priority in boost but im having no luck. Im getting a bad handle error (type 6) from the GetLastError function. I though native_handle() returned the handle for the thread?

Any one know how to do this?

void baseThread::applyPriority(uint8 priority)
{

#ifdef WIN32
    if (!m_pThread)
        return;

    BOOL res;
    HANDLE th = m_pThread->native_handle();

    switch (priority)
    {
    case REALTIME   : res = SetPriorityClass(th, REALTIME_PRIORITY_CLASS);      break;
    case HIGH       : res = SetPriorityClass(th, HIGH_PRIORITY_CLASS);          break;
    case ABOVE_NORMAL   : res = SetPriorityClass(th, ABOVE_NORMAL_PRIORITY_CLASS);  break;
    case NORMAL     : res = SetPriorityClass(th, NORMAL_PRIORITY_CLASS);            break;
    case BELOW_NORMAL   : res = SetPriorityClass(th, BELOW_NORMAL_PRIORITY_CLASS);  break;
    case IDLE       : res = SetPriorityClass(th, IDLE_PRIORITY_CLASS);          break;
    }

    if (res == FALSE)
    {
        int err = GetLastError();
    }

#endif
}

edit: Final code:

void baseThread::applyPriority(uint8 priority)
{

#ifdef WIN32
    if (!m_pThread)
        return;

    BOOL res;
    HANDLE th = m_pThread->native_handle();

    switch (priority)
    {
    case REALTIME       : res = SetThreadPriority(th, THREAD_PRIORITY_TIME_CRITICAL);   break;
    case HIGH           : res = SetThreadPriority(th, THREAD_PRIORITY_HIGHEST);         break;
    case ABOVE_NORMAL   : res = SetThreadPriority(th, THREAD_PRIORITY_ABOVE_NORMAL);    break;
    case NORMAL         : res = SetThreadPriority(th, THREAD_PRIORITY_NORMAL);          break;
    case BELOW_NORMAL   : res = SetThreadPriority(th, THREAD_PRIORITY_BELOW_NORMAL);    break;
    case IDLE           : res = SetThreadPriority(th, THREAD_PRIORITY_LOWEST);          break;
    }

#endif
}