fetchAndLock returns bad request

Yes, the REST API is working and I’ve deployed and completed processes/tasks before. I can carry out a GET request to /engine-rest/external-task?processInstanceId=xxxx and retrieve the external task. I can’t however carry out a fetchAndLock POST request, and because I can’t fetchAndLock the task, when I try to complete the task I receive:

{
“type”: “RestException”,
“message”: “External Task 5557710f-fef4-11e9-b440-d43b04212cfe cannot be completed by worker ‘aWorker’. It is locked by worker ‘null’.”
}