Problem with sorted <map> when a value is null

Question

Hi,

I'm trying to persist a SortedList<DateTime, double?> with NHibernate 2.0. When an item's value (not key) is null, that item is not persisted to the database.

Class snippet:

public class TimeSeries{
    ...
    public TimeSeries(){
        Data = new SortedList();
    }
    public virtual IDictionary Data { get; private set; }
    ...
}

Mapping snippet:

...
    <map name="Data" cascade="all" lazy="true" sort="natural" collection-type="sorted-list">
        <key column="ID"/>
        <index type="Date" column="Period"/>
        <element type="double" column="Value" not-null="false"/>
    </map>
...

Usage snippet:

var series = new TimeSeries();
series.Data.Add(new DateTime(2000, 1, 1), 1);
series.Data.Add(new DateTime(2000, 1, 2), null);
series.Data.Add(new DateTime(2000, 1, 3), 3);
repository.Add(series);

The first and third items are persisted, but not the second. If I change the double value from a null value to a non-null value, it saves fine.

Any idea how I can save the null values? I'm using MySql and the database schema allows for nulls.

Thanks, Marcus