Crash on setWindowFlags()

The following app crashes:

#include <QWidget>
#include <QApplication>
#include <QDialog>
#include <QVBoxLayout>

int main(int argc, char **argv)
{
QApplication a(argc, argv);
QWidget w;
QDialog d;
d.setWindowFlags(d.windowFlags() & ~Qt::Window);
QVBoxLayout *l = new QVBoxLayout(&w);
l->addWidget(&d);
w.show();
return a.exec();
}