Empty "cells" in a pivot table

I have a pivot table that displays the sum of closed Opportunity revenue with the columns being the closing week of the quarter and the rows are the sales reps.  Management now wants to also display the average closed revenue in addition to the weekly totals.  If a sales rep doesn't close an Opportunity in a week, the pivot table cell for that week is blank, not zero.  Calculating an average ignores the blank cells, so the average value is invalid.
Is there a way to display a blank cell as zero?  Is there a better way of meeting this need?