Increment (sum) two fields in Opportunity Entity (CRM Online 2011)

Hi Everyone:

I have two fields in CRM Online 2011 that I am trying to sum into a 3rd field.

Field 1 is the Estimated Revenue Field that has been renamed as service revenue

Field 2 is a custom field (currency) called product revenue

Field 3 is a custom field (currency) called total revenue

When I build the workflow I am using the increment by into the total revenue field, so in the total revenue field I see: Increment by {Service Revenue(Opportunity0;Product Revenue{Opportunity}).  Yet the field does not update.

Is this the correct use case?  See attached screenshot.