cart.setTransactionBodyField: Cannot read properties of undefined error

I have a script that sets necessary transaction body fields during the checkout process that seems to have stopped working. The relevant field is applied to sales, displayed in website, has its value stored, is a checkbox, and its default access level is edit. However, when cart.setTransactionBodyField is called, I am told it cannot read properties of undefined even though I am passing in an object of the same structure as the reference material.

To make sure it wasn't my formatting, I copied the actual example for the reference and received the same errors. I also tested another cart method, addLine to make sure that my cart was valid. I was able to add an item to my cart, but then when I try to set a transaction body field, it's saying the object I am passing it is undefined.