.Cacheable().Fetch() throws 'Exception occurred getter of xxx'

This query:
IList<ParentObject> pos = session.Query<ParentObject>().Cacheable().Fetch(po => po.ChildObjects).ToList();

throws this exception:
NHibernate.PropertyAccessException: Exception occurred getter of CacheableFetchTest.Entity`1[[System.Guid, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].Id ---> System.Reflection.TargetException: Object does not match target type..

For a detailed discussion, including full exception stacks, please see:
http://groups.google.com/group/nhusers/browse_thread/thread/dfdbe987d8450b0a/cb812c856e07ee0a