Unified toolbar becomes black when adding a WA_NativeWindow widget

#include <QApplication>
#include <QMainWindow>
#include <QToolBar>
#include <QLabel>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QMainWindow w;
    w.setUnifiedTitleAndToolBarOnMac(true);
    QToolBar *bar = new QToolBar();
    QLabel *l = new QLabel("Test");
    l->setAttribute(Qt::WA_NativeWindow);
    bar->addWidget(l);
    w.addToolBar(bar);
    w.show();
    return a.exec();
}