openGL selection buffer returns 0

We are using the openGL selection buffer and picking routine for our scalable interface. The problem is that we often recieve the top hit (closest to the camera) returning an id of 0, when none of the objects have pushed a name id of 0. The 0 hit also seems to be disabling the other name ids, because we can see the objects when rendered in selection mode, but it doesnt even put their names on the names list.

Does anyone know what the cases are for a name in the selection buffer to be 0? We have checked all the official documentation, and nowhere does it mention a problem of this kind.