Maemo5InformationBox shows as black rectangle when a button style is applied

If I display a Maemo5InformationBox on a QMainWindow to which a style for QPushButton is applied, the information box shows as a black rectangle.

The problem can be reproduced by executing the example at:

http://doc.qt.nokia.com/qt-maemo-4.6/maemo5-informationbox.html

And replacing main.cpp with the code below:

#include "examplewidget.h"

#include <QApplication>
#include <QMainWindow>

int main(int argc, char *argv[])
{
QApplication app(argc, argv);

ExampleWidget w;
QMainWindow win;
win.setCentralWidget(&w);
win.setStyleSheet("QPushButton

");
win.show();

return app.exec();
}