Getting the Rep name with highest Sale -Using Rank

[cognos05]

 Hi ,
I was using topcount to get the rep with highest sale, So I faced an issue like if two reps have same sales Max value in that case  only one rep was returned.

I am trying a different approach of using the below expression.

filter([Representative],rank(Tuple(currentMember([Distributor Sales].[Product].[Product]),[Qty])within set [Representative])<=1)

But I am getting an error like , the operand type is not valid for the operator 'tuple','mdxwriter.cpp'.

Cant I not use rank function with tuple. Is there a work around.

Please advise,

Thanks,
Nithya