Receipt Fetch/Delete Endpoint issue

I am currently trying to implement receipts into a Python package that I have created. I am having a bit of difficulty with receipts however.

I have created a receipt only populating the properties that the API documentations states are mandatory. I can see the receipt associated with the transaction in the APP but when I try to fetch or delete the receipt using the API I get the following error

{"code":"bad_response.marshaling","message":"Failed to marshal proto receipt to DTO","params":{"amount_currency":"GBP","client_id":"xxx","external_id":"ABC123","tax_currency":"","user_id":"xxx"},"retryable":{}}

Obviously I have masked the private data in the fields.For a delete I get a 400 status response and for a get I am getting a 406 http response.

Going by the error I suspect the structure of the receipt is wrong but not sure how. Has anyone come across this behaviour and identified the cause?

The following is the payload I used when creating the receipt (again with private data removed):

    "transaction_id": "tx_123ABC",
    "external_id": "ABC123",
    "total": 665,
    "currency": "GBP",
    "taxes": [],
    "payments": [],
    "merchant": {},
    "items": [
        "amount": 665,
        "currency": "GBP",
        "description": "testing receipts",
        "quantity": 1,
        "tax": null,
        "unit": null,
        "sub_items": []

I have actually just found the issue. The API doesnt play nicely with null values for items that are not required. The Monzo app is OK with it but the API will create the receipt but not allow a fetch or delete.