QML Singletons are not available inside QQmlExpressions

class TestClass : public QObject
{
  Q_OBJECT
  Q_PROPERTY(QJSValue func READ func WRITE setFunc);
  // ...
};

engine->rootContext()->setContextProperty("testClass", new TestClass);
QQmlExpression expr(engine->rootContext(), 0,
                    "testClass.func = function () { Singleton.func(); });
expr.evaluate();