QPainter setOpacity 1.0 makes widget dissapear

According to documentation QPainter setOpacity accepts a qreal between 0.0 and 1.0.

When I render my widget with opacity 1.0, it becomes transparent. Setting opacity to 0.999 instead works fine.

I asume something could be wrong with handling qreal type. If my 1.0 is considered out of range, I think it should fallback to the maximum value, not to the minimum.
 

QPainter painter(this);     
painter.setOpacity(1.0);