Access custom record from workflow action

Scenario:

- built a custom record with two columns: Name and Number of Days (Integer Number). Record is called Follow-up Categories
- created custom transaction field (List/Record) called FU Category that uses Follow-up Categories
- built a Set Field Value workflow action that executes on Field Change event for FU Category on the transaction. The action should set a custom Date field to today's date + the integer in the custom record. Here's the Value formula:

nlapiDateToString(nlapiAddDays(new Date(), {custbody_followupcategory.custrecord_numberofdays}))

It almost works, but instead of pulling the integer from Number of Days, it pulls the internal ID integer for the respective record. In other words, if I choose the first-entered record in the dropdown, which should have a value of 10, it adds 1 to today's date. I'm certain it's looking at the correct record/row each time, but it's simply not returning the specified column.