discriminator-value cannot be set to null

[tohidemyname]

I am porting a Java hibernate project to C#. The Java project has an annotation:

@DiscriminatorValue("null")

I port it to hbm.xml:

<class name="xxx" discriminator-value="null" entity-name="xxx">

I got an exception:

NHibernate.Exceptions.GenericADOException:“could not execute batch command.[SQL: SQL not available]”

It looks like that NHbernate does not support null discriminator-value?