how tu use aggregate functions in queries

helo,
can any body let me know how tu use aggregate function in queries fired with command object and datareader object with microsoft access database.
in my project, i m firing a query on a table named "subscribed_user_details" on the field sr_no (its serial number) as following....
"select max(sr_no) as sr from subscribed_user_details group by sr_no"
but when i print using response.write(objdatareader("sr").tostring) the following error occurs :

No data value available for specified column/row.
Please help me where i m going wrong..its very urgent