Using an interface for subquery in NHibernate

I usually query interfaces using DetachedCriteria in NHibernate:

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

      

        
        
        
      

    

And it works great.

Now I want to create a subquery for the child object like this:

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

      

        
        
        
      

    

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

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

      

        
        
        
      

    

This works if my DetchedCriteria for the child is:

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

      

        
        
        
      

    

but not it for the interface (as above). In this case, I get an exception:

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

      

        
        
        
      

    

Is this something it should have been, or am I missing some configuration?