Queries :: Replace Null With Currency Zero?

I have always relied on Design View to create my queries, usually with great success, but in so doing, I have not become familiar with SQL view, so can't solve this problem:

I have a very simple query whose only function is to sum the payment amounts received for each contract number:

SELECT Payments.[Contract Number], Sum(Payments.[Payment Amount]) AS [SumOfPayment Amount]
FROM Payments
GROUP BY Payments.[Contract Number];

It is then used in subsequent queries to create an accounts payable report. It works great as long as at least one payment has been received on the contract, but if NO payments have ever been received, the Sum is actually null, so that record doesn't get included. I need a way for this query to return zero or currency zero 0.00 for all contracts with no payments yet, so those contracts will be included and the next query in line will see a zero.

In my search, it seems I need to make use of the IsNull function in my SQL (the check expression would be the sum of payments, the replacement value would be zero) OR I need to include a WHEN or WHERE in my SELECT statement above? I just don't know enough about SQL to make this happen .