Acme Exercise - Part 9 PDF - Record Relationship Not Working from Expression

I am completing the exercises in the PDF part 9 of the Acme Auto application, and the relationship between AX Vehicles and AA Vehicle Category is set up correctly. There is relationship set up of many AA Vehicle Category to one AX Vehicles. I am able to write a query expression that fetches the records from AX Vehicles, and when I test it, I correctly get the record referenced from toCategoryRecord. I can see the category name in the results. The AX Vehicles record has a category ID of 5, which exists in the AA Vehicle Category records (Convertible).

And exactly as the PDF says, for the "Category" field of the interface, in "Display Value", I am entering exactly what the PDF says to put:

a!defaultValue(
 ri!vehicle[recordType!AX Vehicle.relationships.toCategoryRecord.fields.value],
 "–"
)

But this relationship from an expression always returns null instead of "Convertible" as doing a query with the relationship does.

I am completely and utterly stuck. As far as I can tell, I have followed all of the directions correctly from PDFs part 1 to 8.

I am in dire need of help here to get unstuck. Any ideas of what the problem could be?