NHibernate 2.0.1 GA has a bug that show up when you have generate_statistic = true and the use of the 2nd level cache.

Details

If you have generate_statistics = true, and use the 2nd level caching and the query returns no result, an exception is generated.

All three conditions must be present for the bug to appear. This is a bug that has since been fixed and will be included in NHibernate 2.1