CompositionMode_Screen wrong with drawImage

When using QPainter::drawImage with the CompositionMode_Screen mode there seem to be an off by one / rounding ( ? )  error.

The result is always (if < 255) more than one would expect.

For example:

QImage dst(4, 4, QImage::Format_Grayscale8);
dst.fill(0);
QPainter p(&dst);
p.setCompositionMode(QPainter::CompositionMode_Screen);
qDebug() << "Original color is" << dst.constScanLine(0)[0];

QImage src(4, 4, QImage::Format_Grayscale8);
src.fill(0);
p.drawImage(0, 0, src);
p.end();
qDebug() << "New color is" << dst.constScanLine(0)[0];

 Results in

Source color is 0
Color is 1

while the expected result when screening with zero would be the same as the destination color (0 in this case).

(See Wikipedia and also the documentation: "whereas screening a color with black leaves the color unchanged")
This behavior is the same on all tested Image formats (GrayScale8,  RGB, ARGB_PREMUL).

When replacing the drawImage call with fillRect with the same color the result is as expected / correct.

(I sadly didn't found the drawImage on RasterImages implementation in QTs code otherwise i could bee more precise. Pointers to the correct code path would be welcome tho)