just discovered MoI, i like it already, but i'm having problems creating a shape.

My plan is to create a wheel, to mill out on a CNC machine, so i extrude a circle, that works, then i create a wedge-shape, which is also extruded, but then when i want to subtract it from the wheel, nothing happens, it does work however with polylines, or solids from the menu.. am i overlooking something?