More_like_this via Nest for document results in 500 error from ES 6.2.2

[radleta (Richard Adleta)]

We upgraded from ES 5.6 to ES 6.2.2 and started having an issue using .NET Nest 6.1.0 to call ES for more_like_this queries. This use to work against ES 5.6 but fails with ES 6.2.2.

The more_like_this are done via document and it appears the "_id" field is serialized into the request as with null for value. This results in the below error.

Example call.

{
  "size": 250,
  "sort": [
    {
      "_score": {
        "order": "desc"
      }
    }
  ],
  "_source": {
    "includes": [
      "mypkid"
    ]
  },
  "query": {
    "function_score": {
      "query": {
        "bool": {
          "must": [
            {
              "more_like_this": {
                "fields": [
                  "mytitle",
                  "mybody",
                  "myalttext"
                ],
                "like": [
                  {
                    "_index": "myindex",
                    "_type": "mydoctype",
                    "_id": null,
                    "doc": {
                      "altfkid": 0,
                      "mytitle": "title goes here",
                      "fkid": 0,
                      "mybody": "document text goes here",
                      "myalttext": "alt text goes here"
                    }
                  }
                ]
              }
            }
          ]
        }
      },
      "functions": [
        {
          "filter": {
            "multi_match": {
              "query": "look for these words",
              "fields": [
                "myalttext",
                "mytitle"
              ]
            }
          },
          "weight": 1
        }
      ]
    }
  }
}

Error result.

{
    "error": {
        "root_cause": [
            {
                "type": "illegal_state_exception",
                "reason": "Can't get text on a VALUE_NULL at 1:290"
            }
        ],
        "type": "illegal_state_exception",
        "reason": "Can't get text on a VALUE_NULL at 1:290"
    },
    "status": 500
}