Dialog without button does not close()

I tried to use a message box without button.

Please consider the following demo snippet:

QMessageBox* d = new QMessageBox(this);
d->setText("Hello");
// d->setStandardButtons(QMessageBox::Ok);
d->setStandardButtons(QMessageBox::NoButton); // Dialog does not close
d->open();
QTimer::singleShot(3000, d, &QMessageBox::close);

Using the "QMessageBox::Ok" line, the dialog closes after 3 seconds. Using the "QMessageBox::NoButton" line the dialog stays open. This is probably not how it should behave. Even a dialog without buttons should allow to be closed programmatically.