Using 2 Sum Functions In Multiple Join Query.

I have a problem with using two Sum() functions in a multiple join query. It is an Access database that is accessed by VB6.

These are my tables with some sample records:-

Payment(payID, payCustID, payDate, payAmount)
1, 4, 30/09/2004$5.00
2, 4, 30/09/2004$120.00
3, 4, 30/09/2004$20.00

Customer(custID, custName)
1, steven
2, julie
3, blake
4, kate

Invoice(invID, invCustID, invDate, invAmount)
1, 1, 10/09/2004, $50.00
2, 2, 11/09/2004, $50.00
3, 1, 11/09/2004, $150.00
4, 3, 10/09/2004, $20.00
5, 4, 10/09/2004, $5.00
6, 4, 11/09/2004, $120.00

This is my query:-
SELECT DISTINCTROW Customer.custName, Sum(Invoice.invAmount) AS SumOfinvAmount, Sum(Payment.payAmount) AS SumOfpayAmount
FROM (Customer LEFT JOIN Invoice ON Customer.custID = Invoice.invCustID) LEFT JOIN Payment ON Customer.custID = Payment.payCustID
GROUP BY Customer.custName, Customer.custID;

Result:-
custName SumOfinvAmount SumOfpayAmount
blake $20.00
julie $50.00
kate $375.00 $290.00
steven $200.00


The resulting SQL query returns incorrect invoice and payment totals for Kate. The Sum() function has duplicated. Can anyone tell me how I would change my query to fix this problem?


Thanks Steven