BlankValue error when I am providing field value

I am trying to make a new request with the requests API, but am always getting this error:

{"error":"RecordInvalid","description":"Record validation errors","details":{"base":[{"description":"XXX: cannot be blank","error":"BlankValue","field_key":XXXXX}]}}

For some context, here is the request I am submitting to ..

{
    "request": {
        "requester": {
            "name": "Name",
            "email": "email@email.com"
        },
        "comment": {
            "body": "this is a test"
        },
        "subject": "TEST REQUEST - PLEASE IGNORE",
        "custom_fields": [
            {
                "id": "XXXXX",
                "value": "one of the tag values"
            }
        ]
    }
}

The field in question is defined as a drop-down field and is listed as required to solve a ticket. However, I am making sure that the value is one of the tag values specified in the admin but still am getting this error.