使用nhibernate.linq按子类型筛选实体

给定此继承映射:

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
    <class name="User" table="[User]" abstract="true">
        <joined-subclass name="SubUser1" table="SubUser1">
            <key column="UserId" />
            ...
        </joined-subclass>
        <joined-subclass name="SubUser2" table="SubUser2">
            <key column="UserId" />
            ...
        </joined-subclass>
        <joined-subclass name="SubUser3" table="SubUser3">
            <key column="UserId" />
            ...
        </joined-subclass>
    </class>
</hibernate-mapping>

如何查询 SubUser2 和 SubUser3 ?我意识到我可以做到:

session.Linq<User>().OfType<SubUser2>()

但这只允许我按单个类型进行过滤。我试过这个:

session.Linq<User>().Where(user => user is SubUser2)

但这导致了这个错误:

could not resolve property:  of: User

对于如何表达对多个子类型的查询有什么想法吗?