QMutex locked by unknown thread

Question

In the following code, I get an ASSERT in unlock (), because lockingThreads (a QList<QThread*>) does not contain the unlocking (current) thread. Examining lockingThreads, I find an "ntdll" thread that I have no idea what it does and only disassembly can be accessed which doesn't provide useful information. The problem is very hard to reproduce, so thanks for any hint (probably a stupid twist in this logic).

Edit: the mutex is recursive.

MyMutex::MyMutex (QMutex::RecursionMode mode)
: QMutex (mode)
, lockCount (0)
{
}

MyMutex::~MyMutex ()
{
    ASSERT (lockingThreads.isEmpty ());
}

void MyMutex::lock ()
{
    lockCount++;
#if MYDEBUG
    // grab thread before lock so we can identify it if it deadlocks here
    QThread* thread = QThread::currentThread ();
    if (!isRecursive ())
    {
        ASSERT (!lockingThreads.contains (thread));
        if (!lockingThreads.contains (thread))
            lockingThreads += thread;
    }
    else
        lockingThreads += thread;
#endif

    QMutex::lock ();
}

void MyMutex::unlock ()
{
#if MYDEBUG
    QThread* thread = QThread::currentThread ();
    ASSERT (lockingThreads.contains (thread));
    lockingThreads.removeOne (thread);
    if (lockingThreads.contains (thread) && !isRecursive ())
        ASSERTFALSE;
#endif
    QMutex::unlock ();
    lockCount--;
}