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Code that breaks & Exception stack trace: https://gist.github.com/2837045
Prior to 3.3.0 this ICompositeUserType worked correctly.
Have checked with NH3.2.0, NH3.3.0 and current trunk - all works as expected in xml and mapping by code.
As Justin said it may be a bug in FNH.
Playing with this more has lead me to believe it is an issue with FluentNHibernate not generating the *.hbm.xml correctly.
Code that breaks & Exception stack trace: https://gist.github.com/2837045
Prior to 3.3.0 this ICompositeUserType worked correctly.