Aggregate sub query help

Hey Guys,

I'm having a hard time figuring out this problem... I usually write 2 queries for something like this but am feeling adventurous today. I have simplified my code to where the error is:

Description:
my website had a sale on '2013-04-25' and I want to see how many customers made purchases and of those customers how many new customers there were (startdate = '2013-04-25')
*I am looking to do this in the subquery if its possible.


select
count(distinct(p.accountnumber)) as all_buyers,
count(distinct(select distinct cc.accountnumber from customers cc
where cc.startdate = '2013-04-25'
and cc.accountnumber = p.accountnumber)) as new_buyers
from purchases p
join customers c on p.accountnumber = c.accountnumber and c.websitecode = 10
where p.datesettled = '2013-04-25'
go

>[Error] Script lines: 1-10 -------------------------
An expression starting with "ACCOUNTNUMBER" specified in a SELECT clause, HAVING clause, or ORDER BY clause is not specified in the GROUP BY clause or it is in a SELECT clause, HAVING clause, or ORDER BY clause with a column function and no GROUP BY clause is specified.. SQLCODE=-119, SQLSTATE=42803, DRIVER=3.53.71


Thanks in advance for the help!