How to create a blueprint that extends PhysicaVolume

I created a C++ class that extends the physicsVolume. And I can add it into the scene by editor. But I can’t create a blueprint that extends this C++ class. The error is “default_xxxx_c is not valid”. That’s the code.
UCLASS(Blueprintable)
class xxx_api AWaterVolume : public APhysicsVolume
{
GENERATED_BODY()
}