Audio file properties "discNumber" and "composer" not working as expected

Querying audio file properties returns invalid results for the properties
QDocumentGallery::discNumber
QDocumentGallery::composer

Here's some example code that I'm using:

QStringList GalleryScanner::gAudioPropertyNames = QStringList()
<< QDocumentGallery::filePath
<< QDocumentGallery::albumTitle
<< QDocumentGallery::albumArtist
<< QDocumentGallery::artist
<< QDocumentGallery::composer
<< QDocumentGallery::audioBitRate
<< QDocumentGallery::audioCodec
<< QDocumentGallery::channelCount
<< QDocumentGallery::description
<< QDocumentGallery::discNumber
<< QDocumentGallery::trackNumber
<< QDocumentGallery::duration
<< QDocumentGallery::genre
<< QDocumentGallery::keywords
<< QDocumentGallery::comments
<< QDocumentGallery::performer
<< QDocumentGallery::producer
<< QDocumentGallery::sampleRate
<< QDocumentGallery::title;

QDocumentGallery* gallery = new QDocumentGallery();
QGalleryQueryRequest* query = new QGalleryQueryRequest(gallery);
query->setRootType(QDocumentGallery::Audio);
query->setPropertyNames( gAudioPropertyNames );
query->execute();
while (query->state() == QGalleryAbstractRequest::Active)
query->waitForFinished(100);
QGalleryResultSet* result=query->resultSet();

int disckey = result->propertyKey(QDocumentGallery::discNumber);
int discnumber = result->metaData(disckey).toInt();
// wrong: discnumber is always 1
int composerkey = result->propertyKey(QDocumentGallery::composer);
QString composer = result->metaData(composer).toString();
// wrong: composer is always empty