Deleting an object that references itself with NHibernate 2.0.1.GA

Question

I've recently upgraded from NHibernate 1.2 to NHibernate 2.0.1.GA and an issue has occurred attempting to delete an object that references itself.

The mapping file:

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" ..>
    <class name="Account" table="Account" polymorphism="explicit">
        <id name="ID" column="ID">
          <generator class="identity" />
        </id>

        <many-to-one name="CreatedBy" column="CreatedBy" class="Account" />
        <property name="CreatedOn" column="CreatedOn" />
        <many-to-one name="ModifiedBy" column="ModifiedBy" class="Account" />
        <property name="ModifiedOn" column="ModifiedOn" />
    </class>
</hibernate-mapping>

If ModifiedBy references itself, then in NHibernate 2.0.1.GA a GenericADOException occurs saying it could not Update Account "Cannot insert the value NULL into column 'ModifiedBy'".

Why doesn't NHibernate just execute a DELETE to the Account table rather than first updating the ModifiedBy property to NULL? Am I missing something from the mapping file?