Using qt's graphicsitem cast fails on QGraphicsPolygonItem

Question

I have written code in my implemented QGraphicsScene to cast into a custom class from what QGraphicsScene::itemAt returns. Interestingly, qgraphicsitem_cast always returns zero, but using dynamic_cast works fine. Why is this?

void VScene::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    // ...

    QGraphicsItem *item = itemAt(event->scenePos(), QTransform());
    if (item->type() == TypeVModuleItem) {
        // This returns 0
        VModule *module = qgraphicsitem_cast<VModule *>(item);
        // This works
        module = dynamic_cast<VModule *>(item);
        // ...
    }
}

It is worth mentioning that VModule is a QGraphicsPolygonItem, not a QGraphicsItem:

VModule::VModule(QGraphicsItem *parent = 0) : QGraphicsPolygonItem(parent)
{
    // ...
}

Here is the type implementation in VModule:

int type() const { return TypeVModuleItem; }

Does this have any bearing on the failure of the cast? Are there any consequences to using dynamic_cast in this way, such as losing data, that I am not aware of? Thanks in advance for any advice.