ClassCastException[null] when grouping on a particular field

[ankur6971]

I am trying to group the results on a particular field by using the following query:

{
  "from": 0,
  "size": 0,
  "fields": [
    "exitPage.categoryId"
  ],
  "aggs": {
    "check": {
      "terms": {
        "field": "exitPage.categoryId"
      }
    }
  }
}

Elasticsearch server throws this exception:

{
  "error": "ClassCastException[null]",
  "status": 500
}

and that too, on an intermittent basis - sometimes it returns results while sometimes it does not. There is no more descriptive information available in the server log.

Does anyone have a clue to this problem?