QWidget::grabWidget() not working for video files played on Phonon

Launch a Phonon videowidget (inside a QWidget derived class), play a video file (tested with *.avi file) and try to grab a pixmap from the video file:

pMedia = new Phonon::MediaObject(this);
pVideoWidget = new Phonon::VideoWidget(this);
Phonon::createPath(pMedia, pVideoWidget);
pMedia->setCurrentSource(Phonon::MediaSource("myvideo.avi"));
pMedia->play();

QPixmap pixmap = QPixmap::grabWidget( pVideoWidget);
--> returned pixmap is empty i.e. does not contain a capture from the video file

tested also:
QImage image = pVideoWidget->snapshot();
--> returns a null QImage

QPixmap pixmap = QPixmap::grabWindow(pVideoWidget->winId());
--> returned pixmap is empty i.e. does not contain a capture from the video file