What DC should a Skeleton’s Explosive Death ability be?

What DC should a Skeleton’s Explosive Death ability be? I’ve just been using the standard DC for a level equal to the skeleton, which is fine for the Skeletal Champion but that poses some problems for the Skeleton Guard (which is level -1). The Bestiary (at least, Bestiary 1, the only one I have) doesn’t seem to offer guidance on this, but I could be missing it.