Cannot use QSharedPointer with QJSEngine

#include <QtCore>
#include <QtQml>

class Object : public QObject
{
    Q_OBJECT
public:
    Q_PROPERTY(int x READ x WRITE setX)

    Object(int x) : mX(x) {}

    int x() const { return mX; }
    void setX(int x) { mX = x; }
private:
    int mX;
};

class Context : public QObject
{
    Q_OBJECT
public:
    Q_PROPERTY(QSharedPointer<Object> object READ object)

    Context(QSharedPointer<Object> object) : mObject(object) {}

    QSharedPointer<Object> object() const { return mObject; }

private:
    QSharedPointer<Object> mObject;
};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    Q_UNUSED(a);

    QJSEngine engine;

    QSharedPointer<Object> objectPtr(new Object(5));
    Context context(objectPtr);
    engine.globalObject().setProperty("context", engine.newQObject(&context));

    qDebug() << engine.evaluate("context.object.x").toString();

    return 0;
}

#include "main.moc"