Failing the Fortitude save causes you to take damage, starting at "1d10 damage" and increasing with every such save you make. Does this damage not have a type? Can that happen?
I'm not sure that it is supposed to be possible to take untyped damage, but I am also not sure what damage type to give it. I wouldn't want to give it a damage type that could be resisted.
So I would probably just leave it at untyped damage.
I think there are a few spells that do damage transfer that also list that they don't cause typed damage, just reduce HP by a specified amount. Spirit Link and Shield Other are the ones that come to mind immediately.
Apparently untyped damage is intended to actually exist. *shrugs*