nsIDOMCanvasRenderingContext2d embedding in 1.9.2 ?

I'm extending my MozMFC example to handle rendering into a Win32 bitmap. 
I start with the following ...

     nsCOMPtr<nsICanvasRenderingContextInternal> context;
     context = 
do_CreateInstance("@mozilla.org/content/canvas-rendering-context;1?id=2d", 
&rv);
     if (!(NS_SUCCEEDED(rv) && context))
     {
         return 0;
     }
     context->SetDimensions(clientRect.Width(),clientRect.Height());


setDimensions() internally calls InitializeWithSurface(NULL, surface, 
width, height);

Sadly InitializeWithSurface() triggers the following assertion:

NS_ASSERTION(!docShell ^ !mCanvasElement, "Cannot set both docshell and 
canvas element");

Where am I going wrong here? Should there be some additional prior 
initialization of the context?  both docShell and mCanvasElement are null.

Thx++

Jerry