I tried this query:
SELECT Year([InvoiceDate]) & ' ' & Month([InvoiceDate]) AS [Month], IIf(IsNull([customers]![CustomerID]),0,[customers]![CustomerID]) AS Umsatz
FROM Invoices INNER JOIN customers ON Invoices.CustomerID = customers.CustomerID
WHERE (((Invoices.InvoiceDate) Between #1/1/2013#...