"Error: need 'this' for 'bbox' of type 'Bong!(double, 3u)'"

[Enjoys Math via Digitalmars-d-learn]

I'm getting that on the last line of this code:

                auto wh = Vec2([loadSurf.w, loadSurf.h]);
                wh /= 2;
                auto x = wh.plus1Dim(pos[Z]);
                this.bbox = Box3(pos - x, pos + x);

Any ideas?