Need NULL Rows

I'm banging my head trying to figure out why this simple query isn't returning null rows for sales. I'm using a calendar table (MetaDates) that should return all 12 months whether there are sales or not (null). I've tried several combinations without any success. Any help with the below query is much appreciated.

SELECT Customer.Country, MetaDates.[Month], MetaDates.[Year], isNull(SUM(Order_Line_Invoice.Sales),0)
FROM Order_Line_Invoice inner join Customer on customer.custid = Order_Line_Invoice.custid RIGHT OUTER JOIN
MetaDates ON MetaDates.[Date] = Order_Line_Invoice.InvoiceDate
WHERE (MetaDates.[Year] = 2007) AND (Customer.Country IN ('ar'))
GROUP BY MetaDates.[Year], MetaDates.[Month], Customer.Country
ORDER BY MetaDates.[Year], MetaDates.[Month]