How do you create a list collection in NHibernate with a non-nullable index?

I'm using FluentNHibernate to map a bi-directional one-to-many relationship where ordering is important, so I'm using a list:

        HasMany(x => x.Children)
            .AsList(index => index.Column("CHILD_INDEX"))
            .KeyColumn("PARENT_ID")
            .Not.LazyLoad()
            .Access.CamelCaseField()
            .Cascade.AllDeleteOrphan();

On the other side, it's mapped like this:

        References(x => x.Parent)
            .Column("PARENT_ID")
            .Not.Nullable()
            .Fetch.Join()
            .Not.LazyLoad()
            .Cascade.SaveUpdate();

Now in my actual database, the CHILD_INDEX column is not nullable. However, when NHibernate persists the child elements during an insert, it doesn't insert the CHILD_INDEX column. It then performs an UPDATE to set the CHILD_INDEX. This is odd to me, as it certainly has the index value when it does the insert.

Is there a way to tell NHibernate to write the index column during the insert?