QScreen::grabWindow(0, ...) isn't working on second display

#include <QApplication>
#include <QDebug>
#include <QColorDialog>

class QColorDialogTester : public QWidget
{
public:
    void onColor()
    {
        QColor color = QColorDialog::getColor(Qt::yellow,
                                              this,
                                              "Color Dialog",
                                              QColorDialog::DontUseNativeDialog);

        if (color.isValid())
            qDebug() << "Choosen color: " << color.name();
    }

};

int main(int argc, char **argv)
{
    QApplication app(argc, argv);
    QColorDialogTester color_test;
    color_test.onColor();
    return 0;
}

QColor QColorDialogPrivate::grabScreenColor(const QPoint &p)
{
    const QDesktopWidget *desktop = QApplication::desktop();
    const QPixmap pixmap = QGuiApplication::primaryScreen()->grabWindow(desktop->winId(), p.x(), p.y(), 1, 1);
    QImage i = pixmap.toImage();
    return i.pixel(0, 0);
}

QColor QColorDialogPrivate::grabScreenColor(const QPoint &p)
{
    QScreen *screen = QGuiApplication::screenAt(p);
    if (!screen)
        screen = QGuiApplication::primaryScreen();
    const QPixmap pixmap = screen->grabWindow(0, p.x(), p.y(), 1, 1);
    const QImage i = pixmap.toImage();
    return i.pixel(0, 0);
}