WPF exception running animation in another class

Question

Hi
I can run an animation from the main window class but if I try to run the same code from another class in the same namespace I receive an exception in the storyboard begin function
this is the exception:

No applicable name scope exists to resolve the name 'DefaultGroup'.

this is how I set the storyboard target and property

Storyboard.SetTargetName(r3d,"DefaultGroup");
Storyboard.SetTargetProperty(r3d, new PropertyPath("(Visual3D.Transform).
(Transform3DGroup.Children)[2].(RotateTransform3D.Rotation)"));

How can I fix this?
Thanks
Walther