Dependency property not initialized in OnPropertyChanged?

I have made my own class that derives from ModelVisual3D called
'PlayfieldView'. To this class I added a dependency property called
'SlotModel' of type GeometryModel3D. In XAML, I supply a property value
like this:

<gp:PlayfieldView.SlotModel>
<GeometryModel3D
Geometry="{DynamicResource TorusGeometry}"
Material="{DynamicResource ERRasterChecker}"
/>
</gp:PlayfieldView.SlotModel>

In my class, I have overridden OnPropertyChanged and I do get notified
whenever the property is set by the XAML deserializer. The problem is
that when the property is set, the Geometry and Material properies of
the GeometryModel3D instance supplied by the deserializer are both null.

I know the dynamic resources 'TorusGeometry' and 'ERRasterChecker' both
exist and I know they load just fine. If I create a Model3DGroup in XAML
and drop in a GeometryModel3D using those resource names the object
appears as expected.

Why are both geometry and material null when my dependency property is
set (OnPropertyChanged)? I would like to create an array of
GeometryModel3D instances based on the one applied to my dynamic
property and add them as the content of my specialized class at runtime.
I have the code written, but since the geometry and material are both
null, nothing appears in the scene.

Thanks,

Jared