Define query parameters visual studio

I am creating a SSRS report in VS 2008 sp1.

I am trying to run the SP of a certain dataset So I am loading the query designer

Than i am get the normal define query parameters dialog box

I have two columns one for the parameter Name and one for the parameter value

one of the Parameter values of the is nvarchar, so a valid input is e.g. 36,37,38

The query runs but doesn't give any result and when i load the define query parameters dialog box again the parameter value appears as: 363738

The commas were removed from some unclear reason

how do i make the commas to stay?

BTW the the same input works just fine in ssms with the same input.

Here are the headers of the SP

 ALTER PROCEDURE [dbo].[usp_CVM_FollowNewGrou开发者_开发问答ps] 
   @GroupID int
 , @periods nvarchar(max)
 , @numOfPeriodsAhead int
 , @FieldToShow nvarchar(max)
 AS
 BEGIN