Validation error when setting custom field

I'm trying to update a custom field value via the API. I'm using a PUT request to /api/v2/tickets/<ticketID>.json I'm getting a validation error as follows:

{
  "error": "RecordInvalid",
  "description": "Record validation errors",
  "details": {
    "base": [
      {
        "description": "Incident vs. Service Request: needed",
        "error": null,
        "ticket_field_id": 21621964218653,
        "ticket_field_type": "FieldTagger"
      }
    ]
  }
}

The “Incident vs. Service Request” field is not the field I'm updating. I assume there some is configuration that is making this field required when updating a ticket, although I can't figure out what that config might be? The field has “Required to solve a ticket” enabled, but I'm not trying to solve a ticket, and there are other fields with this setting which don't cause the same problem. Can anyone help me out?

Thanks.