How to calculate distinct count per group?

I have Department group and want to count
how many distinctive customers we have in each of them:
I placed this formula in group footer
:=countDistinct(Fields!Customers.Value,"Department")
but it shows me total number of customers in each row.
OlegHope u followed the following procedure:
1. Group on Department.
2. CountDistinct(Fields!Customers.Value, "<group name>")
NOTE: <group name> is the name of the group not the field on which grouping
is done.
Regards,
Saras