problem using parameter with null value

Query selects records where a field has a null value, this returns a number of rows when run in SSMS but when I create in reporting services and use a parameter and assign null value the report returns no records.

I have allow nulls

and allow blanks ticked

I use a dropdown box so user can select null or other value

"select distinct dischargedestination as  dischargedestination , dischargedestination as Name from [case]

union all

select Null as dischargedestination , 'None' as Name"

 

All other values return correct data when selected  but null returns no records?