HasMany's OptimisticLock Options

I'm trying to set turn off the OptimisticLock value on a bag, for example:

public class LoadMap : IAutoMappingOverride<Load> {
    public void Override(AutoMapping<Load> mapping) {
        mapping.HasMany(x => x.Bids).OptimisticLock.None();
    }
}

But I get an exception when creating the session that says:

The string 'none' is not a valid Boolean value.

I checked the NHibernate documentation and it seems to indicate that the proper value for OptimisticLock for collections should be true/false. http://nhforge.org/blogs/nhibernate/archive/2009/04/13/nhibernate-m... Am I doing something wrong in the automapping override?