How do I divide the stardard dev of a series by the average of the same series in a pivot?

I need to find the CoV of booked units.

This is simple outside of a pivot table, as I just add the Booked unit series twice and change the values to Std Dev and Average. Then I can just divide them in a cell next to the pivot table.

The thing is I need to find the Cov for thousands of items across different regions/countries and sort them, updating each month. It would be best to have this Cov calculation directly in the pivot. I would like to create a calculated field that will divide the Std Dev of my "Booked units" series by the Mean of the same series. When I enter "=STDEV.P('Sum of Booked Units')/AVERAGE('Sum of Booked Units')" I just get zero.

cov example.PNG