QPainter dashed lines are wrongly scaled for thin lines

I use a QPainter to paint lines with a custom dash pattern onto a QImage.  This works as long as the line width is at least one pixel.  Otherwise, the dash pattern is scaled wrongly.  The attached small example program demonstrates the issue: It draws four lines with varying line width.  All four lines should have the same dash pattern, but the last pattern, corresponding to the line of widht 0.5, is wrong.