QImageReader: format() does not work if first called after size()

The exemple reproduce the problem

#include <QtGui>
int main(int argc, char *argv[])
{
QCoreApplication app(argc, argv);

QImageReader reader1("test.png");
qDebug() << "format1 " << reader1.format(); //works

QImageReader reader2("test.png");
reader2.size();
qDebug() << "format2 " << reader2.format(); //doesn't works

return 0;
}