Can't find needed Compare To field for Summary Search Custom field

I know I'm just overlooking something dumb, but I can't seem to get this to work.

I have a custom record, called NPS Score.  This record has three fields: Customer (which indicates the parent), Score, and Date.  I want to have a custom field on the Customer record which displays the most recent NPS Score value.

I read the article on creating summary search custom fields.  I created an NPS Score search which has no criteria, a single result of Score (Max) as Ordered by Date, with an available filter on Customer.  The search runs just fine and I can see the most recent score for the customer selected in the filter.  I created a custom field on the Customer.  Value is not stored.  The search is referenced.  The field is the same type as the score field on the NPS Score record.