Problem with creating bindings involving interfaces

Hi,

I'm experimenting with Vala (which looks _very_ promising), trying to 
create bindings for GooCanvas. Here is a snippet of my bindings file:

namespace Goo {
    public interface CanvasModel {
        [NoArrayLength ()]
        public Goo.CanvasItem get_root_item ();
    }

    public class CanvasModelSimple : CanvasModel {

        [NoArrayLength ()]
        public construct ();
    }
}

A test app does essentially this:

using Goo;

...

var model = new Goo.CanvasModelSimple ();
model.get_root_item ();

and results in this error message:

test.vala:41.9-41.21: error: The name `get_root_item' does not exist in 
the context of `Goo.CanvasModelSimple'

Am I missing something?

Cheers,

Dominique Würtz