QMessageBox::question cannot use keyboard shortcuts for Yes & No

QMessageBox::question : The keyboard shortcuts for Yes & No (Y & N) are underlined but not functional. Pressing N / Y / Alt-N / Alt-Y has no effect.

The code is :

QMessageBox::StandardButton answer = QMessageBox::question(this,QObject::tr("The routine was modified"), QObject::tr("Should the routine be saved before exiting?"), QMessageBox::Yes | QMessageBox::No | QMessageBox::Cancel);