QImage::fill() behaves differently to QImage::setPixel() on indexed image, out-of-range

QImage labels(image.width, image.height, QImage::Format_Indexed8);
labels.fill(255); // works
labels->setPixel(x, y, 255); // does not work, message at runtime: QImage::setPixel: Index 255 out of range

Correct code that avoids undefined behaviour:
QImage labels(image.width, image.height, QImage::Format_Indexed8);
labels.setNumColors(255);
labels.fill(255);

I assume fill() should also check the index range to avoid undefined behaviour.