OS X: Close or minimize button hint in modal dialog has no effect

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

QApplication mainApp(argc, argv);

MainWindow *mainWindow = new MainWindow; 
mainWindow->show(); 
QDialog *dialog = new QDialog(mainWindow); // QMainWindow class is parent 

dialog->setWindowFlags(Qt::Dialog|Qt::WindowCloseButtonHint | Qt::WindowMinimizeButtonHint); 
dialog->exec(); 

return mainApp.exec(); 
}