QPrinter::setPaperSize(PaperSize) fails on Mac with non-metric paper

#include <QtGui>

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

  QPrinter printer;

  printer.setPaperSize(QPrinter::A4);
  qDebug() << printer.paperSize() << printer.paperSize(QPrinter::Millimeter);

  printer.setPaperSize(QPrinter::A5);
  qDebug() << printer.paperSize() << printer.paperSize(QPrinter::Millimeter);

  printer.setPaperSize(QPrinter::Letter);  // Absolutely nothing happens
  qDebug() << printer.paperSize() << printer.paperSize(QPrinter::Millimeter);

  return app.exec();
}