You assume that the screen is perfectly absorbing when you use this field. By uses this field you are assuming that the electromagnetic field pass through the material without reflection. Furthermore, you are assuming that you don't have field in the another side of the surface.

For example, if your screen is perfectly reflective, then you need to use the superposition of the incoming field and the outgoing field (reflected). If your screen is perfectly transparent, then you need to use the field in the two side of the surface, getting zero.