QImage.convertToFormat fails to convert from RGB888 to Mono

The method QImage::convertToFormat() fails to convert an image from RGB888 to Mono. The following code returns a Null image.

// image is a QImage in format RGB888QVector<QRgb> colors(2);
image.convertToFormat(QImage::Format_Mono, colors)T

The conversion does work as expected if I do not specify a color vector. Also, the conversion does work as expected if the image is in format RGB32 rather than RGB888. I attach a minimal test case