Check for null values using AutoGenerateWhereClause?

 I'm using a session variable as one of my whereparameters for a LinqDataSource which is being used to populate a GridView.  I'd like this LDS to only return values which have null for one of the other columns.  Current code:
 
        <asp:LinqDataSource ID="ActivitiesDS" runat="server"
            ContextTypeName="DataClassesDataContext" EnableDelete="True"
            EnableInsert="True" EnableUpdate="True" TableName="Activities"
            AutoGenerateWhereClause="true">
            <WhereParameters>
             ...