There were changes to the Teller API which meant the JSON parsing started to fail.
Mainly account_number_last_4_digits became account_number, and the responses no longer consist of some metadata and a data property (instead the latter is the whole response).
I fixed the parsing and made the unit tests pass. However I'm not comfortable at saying this isn't broken in some way since it's been over a year since I wrote code for it and my updates were naive. I wasn't very conscious of the semantics of the code I was altering.