Flat QPushButton and style sheets

It is not possible to set the background color of a flat QPushButton using style sheets.

Example:

QPushButton* button = new QPushButton();
button->setFlat(true);
button->setStyleSheet("QPushButton:flat { background: yellow; }");

The backgronud is set to yellow only when the button is pressed.

If I remove the flat pseudo state in the style sheet, it does not change anything.

But if I set the border to none, the background is set to yellow correctly:

button->setStyleSheet("QPushButton:flat { background: yellow; border: none; }"); //OK