How to apply null condition in Crystal report for Parameter field

Hello Expert,

I had created procedure and run it in SAP B1 through Crystal Report.In this procedure I have applied the condition null for the parameter @docnum.But when I try to run the report it displaying the message that the parameter field is mandatory.Please suggest some solution for it.

Also I want to populate the list of all the Baseref from SAP B1 in the parameter field @Docnum while running the crystal report.

My Procedure is given below....

SELECT @DOCNUM as DOCNUM ,T1.[BaseRef],T0.[DocNum], T0.[DocDate],T0.[cardname],T0.[U_VehcleNo],

T0.[U_DvrName], T1.[Dscription],T1.[Quantity],T1.[unitMsr], T2.[BatchNum], T2.[Quantity],

T1.[WhsCode] FROM Odln T0 INNER JOIN dln1 T1 ON

T0.DocEntry = T1.DocEntry, IBT1 T2 WHERE T2.[BaseEntry] = T0.[DocNum]

AND T2.[ItemCode] = T1.[ItemCode] AND

t0.datasource <>'N' and T1.[BaseRef] = case when @DOCNUM = '' or @DOCNUM is null

then T1.[BaseRef] else @DOCNUM end

regards,

PankajK

IDM Services Ltd.

Nigeria