Empty Params not allowed

I have a Stored Procedure setup. 2 of the variables can be empty or null.

When I put it into report designer I set the param as can be empty.
However when I go to preview it says it can not be empty

The parameters are always re-created on the fly according to your stored procedure’s setup. Thus, changes in the designer to these auto generated parameters are ignored at print time. Currently, you cannot set an auto generated parameter to accept an empty value, unfortunately. We will look at adding this feature in one of the next service packs.

This will be part of the service pack 29.001 and is incorporated already in our nightly build available from the service pack download area. Use allowempty=true to enable the new feature.