Get error Type MIsmatch in Criteria Expression when left join date

hello,
i have a table CustomerOrders in which it has Item, Due as fields.
I'm trying to left join a query (InvoiceItems) in which it has Item (text),OrderDue (date)
Orderdue though is being calculated as OrderDue: DateAdd("d",ORDERS!DUE,"28-Dec-1800") because the ORDERS!DUE is a number and converting to Date

i left join and it does find the results but on the null values, it gives me an #ERROR in the InvoiceItems.OrderDue field.
I'm trying to set up the criteria so that OrderDue is null but i get Type Mismatch in criteria. How cani fix?

SELECT CustomerOrders.Due, InvoiceItems.OrderDue
FROM CustomerOrders LEFT JOIN InvoiceItems ON CustomerOrders.Due = InvoiceItems.OrderDue
WHERE (((InvoiceItems.OrderDue) Is Null));