AdornerLayer.GetAdornerLayer returns null

I've a got a xaml with a Ribbon control and a custom canvas class that allows the user to drag / resize UI elements on the form.

when the ribbon is on the xaml, the AdornerLayer.GetAdornerLayer(this) returns null,
If I remove the ribbon, the code works...
any ideas as I need to add my own adornments to the UIElements on my canvas.

Cheers chaps
John