Why RenderingPrimitives are always invalid in Vuforia

my code is :

   renderingPrimitives = new Vuforia::RenderingPrimitives(Vuforia::Device::getInstance().getRenderingPrimitives());        Vuforia::ViewList& viewList = renderingPrimitives->getRenderingViews();

   for (int i = 0; i <viewList.getNumViews(); i++)    {     Vuforia::VIEW vw = viewList.getView(i);

    Vuforia::Vec4I viewport = renderingPrimitives->getViewport(vw);

 }

but viewport is {0,0,0,0}  

I wait for a long time ,but viewport is still {0,0,0,0}

I have called Vuforia::CameraDevice::getInstance().start()  before I run the Mentioned code。

why