QNetworkRequest::PreferCache does not behave as expected

CacheRequestTest::CacheRequestTest()
{
    manager = new QNetworkAccessManager();
    QNetworkDiskCache *diskCache = new QNetworkDiskCache();
    diskCache->setCacheDirectory("cacheDir");
    manager->setCache(diskCache);

    connect(manager,&QNetworkAccessManager::finished,this,&CacheRequestTest::replyFinished);




}

void CacheRequestTest::testCache(){
    QNetworkRequest request(QUrl(QString("http://qt-project.org")));
    manager->get(request);

    // do a request preferred from cache
    QNetworkRequest request2(QUrl(QString("http://qt-project.org")));
    request2.setAttribute(QNetworkRequest::CacheLoadControlAttribute, QNetworkRequest::PreferCache);
    manager->get(request2);
}

void CacheRequestTest::replyFinished(QNetworkReply *reply) {
    QVariant fromCache = reply->attribute(QNetworkRequest::SourceIsFromCacheAttribute);
    qDebug() << "page from cache?" << fromCache.toBool();

}