VTK to OCC

hello!I want transform "VTK" to "OCC" by  IVtkTools, below code is how i do.

IVtkOCC_Shape::Handle occShape = IVtkTools_ShapeObject::GetOccShape(srcActor);

srcActor is a  vtkActor  object pointer .this object is no problem as i can see a entity by set this vtkActor in a scene.  

But what my question is when my code excute at this position ,I get  occShape is NULL,So this transformation is failed. 

who can help me to sove this question? thank you advance.