Elasticsearch nested type update

gistfile1.js

Works:
{
  "script": "ctx._source.comments = comments",
  "params": {
    "comments": [{
      "id": 123,
      "body": "boom"
    }]
  }
}

Does not work for nested type:
{
  "script": "ctx._source.comments += comment",
  "params": {
    "comment": {
      "id": 123,
      "body": "boom"
    }
  }
}
Throws MapperParsingException[object mapping for [test] tried to parse as object, but got EOF, has a concrete value been provided to it?]

Mapping:
{
  "test" : {
    "properties" : {
      "author" : {
        "type" : "string"
      },
      "comments" : {
        "type" : "nested",
        "properties" : {
          "body" : {
            "type" : "string"
          },
          "id" : {
            "type" : "long"
          }
        }
      }
    }
  }
}