How do I set the default "Select All" in SSRS?

Possible Duplicate:
Select All as Default for Multivalue

I have a dropdown parameter in SSRS that is populated with the following request:

select ID, name from accounts where LK_RecordStatusID = 1 order by Name asc

      

        
        
        
      

    

I need it to be a multi-value parameter. In SSRS, I can do this. However, I also cannot let it be null. However, I would like it to have a default value of "Null", which corresponds to the "Select All" option.

I was looking online and I see something about using -1 as an alternative to null? I can't seem to specify a null value. Any advice is appreciated, thanks