outside operation and routing costing

Hi all:

Hope you all are doing well, currently I´m performing a discrete manufacture setup, whitin that setup I am doing an outside operation as the only step in the routing, since the labor will be made in another company, but the cost of the work order has to billed to the original company that made the work order,

So no material or labor is being charged in the original company wo, however when I perform 02 average costing for the BOM (which only includes the outside operation ítem cost type D1) Im still getting a charge for materials (A1), how is that posible since I did not include any materials on the BOM.