QImage and QImageWriter does not save PNG image

The following code fails (It works with a different formats, for example TIF, and works with PNG in Qt 5.4.1):

 
QImage image;
...
image.load("test.png");
image = image.convertToFormat(QImage::Format_ARGB32);//optional
bool couldSave = image.save("test2.png"); //couldSave returns false
Q_ASSERT(couldSave); //This fails
...
// Or so
QImageWriter writer("test2", "PNG");
bool r = writer.write(image); //This fails
qDebug()<< r;// r - false
qDebug() << writer.errorString();//An unknown error occurred