ssrs 2005 multi value parameters allow null values

Create dataset and map with parameter and set multi value.
Like
Select UserId, UserName from user

Then create other data set same feild names and Set as a Default data set
like -------------
Select '11111111-1111-1111-1111-111111111111' as UserId, '' as UserName

Validate your sql.....

IF (@User ='11111111-1111-1111-1111-111111111111' )
.....

Now it works fine :)