SSRS Dataset Filter Operator

Hi --

I have a Dataset which returns data based on a multi select parameter. It uses a filter to achieve this and works as expected (based on the IN Operator).

Now I have to update the report so it also returns records for this parameter value which are NULL - Is it possible to achieve this using an expression/filter within the report, or do I have to get the DBA's to review and update the back-end stored procs?

Thanks