nHibernate one-to-many mapping question

I am getting this exception when mapping one-to-many relation. my mapping xml looks like this:

<set name="-----" generic="true" inverse="true" cascade="a开发者_开发技巧ll" lazy="false">
      <key column="Id" />
      <one-to-many class="-----------,---------" />
</set>

And,

<many-to-one name="------" column="Id" not-null="true" class="---------,-------" />

and the Exception:

------ Test started: Assembly: ----------.dll ------

TestCase '-------.GetSchema' failed: NHibernate.MappingException : Could not determine type for: Iesi.Collections.Generic.ISet`1[[-----------, --------, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]], Iesi.Collections, Version=1.0.1.0, Culture=neutral, PublicKeyToken=aa95f207798dfdb4, for columns: NHibernate.Mapping.Column(--------)
   at NHibernate.Mapping.SimpleValue.get_Type()
   at NHibernate.Mapping.SimpleValue.IsValid(IMapping mapping)
   at NHibernate.Mapping.Property.IsValid(IMapping mapping)
   at NHibernate.Mapping.PersistentClass.Validate(IMapping mapping)
   at NHibernate.Mapping.RootClass.Validate(IMapping mapping)
   at NHibernate.Cfg.Configuration.Validate()
   at NHibernate.Cfg.Configuration.BuildSessionFactory()
   at in 
0 passed, 1 failed, 0 skipped, took 4.20 seconds (NUnit 2.5.2).

Please tell me whats going on here. Thanks, Reddy

Edit: Sorry folks, It's a typo, i found it after posting my question... weird....

In my case it was not a typo, I had a class RuleInfo that had a field of type ActionInfo.

I managed to solve my problem by adding [Serializable] to ActionInfo class.

The funny thing is that everything worked fine while using ActionInfo, i.e. I could use my unit tests to CRUD it on the DB.

I could also CRUD RuleInfo class without the ActionInfo field. After adding the field the error appeared.