Match empty cell in SUMPRODUCT condition

[wprager]

I have a resource planning sheet where I have the following formula (calculating per-resource effort for a particular month):

=SUMPRODUCT((List1[Resource]=$G33)*(List1[[Resource]:[When needed]]="Sept")*List1[Optimistic Effort (days)])

The resource name in cell $G33 will match what is listed in column 'Resource'. Now, some tasks have not been allocated to a resource, so the 'Resource' column will have an empty cell for that particular row, however when I change the formula to this:

=SUMPRODUCT((List1[Resource]="")*(List1[[Resource]:[When needed]]="Sept")*List1[Optimistic Effort (days)])

I get 0, even though there are some tasks with no resource identified.