Exiv2

I am having a lot of trouble correctly using a custom xmp namespace from multiple threads.
Reading the documentation it is apparent that I must call XmpProperties::registerNS and XmpParser::initialize on my main thread. This I do. In my main() function I do this:

Exiv2::XmpProperties::registerNs("http://www.alienskin.com/exposure#", "alienexposure");
    QMutex xmp_mutex;
    Exiv2::XmpParser::initialize(XmpLockUnlock, &xmp_mutex);

where the locking function looks like:

void XmpLockUnlock(void* pData, bool fLock)
  {
    QMutex* mutex_ptr = reinterpret_cast<QMutex*>(pData);
    if (mutex_ptr)
    {
      (fLock) ? mutex_ptr->lock()
          : mutex_ptr->unlock();
    }
  }

The QMutex is taken from the stack in main(), so it should be valid for the run of the program.

There is one thread (not on the main thread ) that reads metadata. There is another thread that occasionally writes xmp metadata.
When the 2 threads both try to operate on metadata at once, there is a crash. These threads are NOT trying to write to / read from the same file.
Here is the relevant portion of the crash log on mac. I have elided some template spew for clarity. It is clearly related to a namespace operation:

0 com.alienskin.exposure7 0x0000000104dbca83 std::__1::__tree<std::__1::__value_type<std::__1::basic_string.....) + 35
1 com.alienskin.exposure7 0x0000000104db97c5 XMPMeta::DeleteNamespace(char const*) + 133
2 com.alienskin.exposure7 0x0000000104da0131 WXMPMeta_DeleteNamespace_1 + 65
3 com.alienskin.exposure7 0x0000000104d8f50c Exiv2::XmpParser::registerNs(...) + 92
4 com.alienskin.exposure7 0x0000000104d909bb Exiv2::XmpParser::encode(...) + 155
5 com.alienskin.exposure7 0x0000000104d981d6 Exiv2::XmpSidecar::writeMetadata() + 118
6 com.alienskin.exposure7 0x0000000104a9b4fe MetaDataIO::MetaDataChanged() + 2766

Could someone help me figure out what I am doing wrong?