Using an interface for a subquery in NHibernate

Question

I normally query interfaces using DetachedCriteria in NHibernate:

DetachedCriteria crit = DetachedCriteria.For<IParent>();

And this works fine.

I now want to create a subquery for a child object thus:

DetachedCriteria subcrit = DetachedCriteria.For<IChild>();

and add it to the criteria like this (kind of, p.Child is actually an alias but I've simplified):

crit.Add(LambdaSubquery.Property<IParent>(p => p.Child.ChildID).In(subcrit));

This works if my DetchedCriteria is for a Child:

DetachedCriteria subcrit = DetachedCriteria.For<Child>();

but not it it's for the interface (as above). In that instance I get an exception:

NHibernate.MappingException: No persister for: Domain.Name.Space.IChild

Is this something that's meant to be or am I missing some config?