QPixmap::grabWindow fails to grab QGLWidget

When grabbing a GLWidget using QPixmap::grabWindow() with glWidget::winId() it fails to grab , though if we use QPixmap::grabWidget() with glWidget it does.

// This does not work
QPixmap::grabWindow(glWidget->winId()).save( "glwidget.png", "PNG" ); //This does work
QPixmap::grabWidget(glWidget).save( "glwidget2.png", "PNG" );

This behavior is found on Window vista.