NHibernate 2.0:

The code that I am executing is for example:

ICriteria criteria = Session.CreateCriteria(typeof(User));
criteria.Add(Expression.Eq("UserName", userName));
criteria.SetCacheable(true);
return criteria.UniqueResult<User>();

After closing the application, instantiation of NHibernate throws an error:
"Key cannot be null.\r\nParameter name: key", and from the log4net output, I deduce it is when the cache is loaded the old session is tried to be called (the uuid is copied from previous run).

2009-03-25 19:15:26.312 [10] DEBUG NHibernate.Caches.Prevalence.PrevalenceCacheProvider - configuring cache in C:\MyApp\bin\Debug\cache.
2009-03-25 19:15:26,375 [10] WARN NHibernate.Impl.SessionFactoryObjectFactory [(null)] - Not found: cb05d7fec9ad45709d3ebbc92b9982fc

Also this number is inside the files in the cache directory (in a serialized session object apparently, for each serialized ICriteria):

0003da0: 0000 0100 0000 050c 0000 0022 4e48 6962 ..........."NHib
0003db0: 6572 6e61 7465 2e49 6d70 6c2e 5365 7373 ernate.Impl.Sess
0003dc0: 696f 6e46 6163 746f 7279 496d 706c 0300 ionFactoryImpl..
0003dd0: 0000 046e 616d 6504 7575 6964 0864 6973 ...name.uuid.dis
0003de0: 706f 7365 6401 0100 0108 0000 000a 0611 posed...........
0003df0: 0000 0020 6362 3035 6437 6665 6339 6164 ... cb05d7fec9ad
0003e00: 3435 3730 3964 3365 6262 6339 3262 3939 45709d3ebbc92b99
0003e10: 3832 6663 0005 0d00 0000 1f4e 4869 6265 82fc.......NHibe

Is this a problem of the cache implementation? or because the NHibernate way it is using the cache or serializes the object?