sort filed is not working in golang chaincode of Hyperledger fabric

Question

The sort field is not working in the go lang chain code.

Here is my query string:

    queryString := `{
        "selector": {
            "dockType": {
                "$or": [
                    "Asset1",
                    "Asset2"
                ]
            },
            "timeStamp": {
                "$gt": "null"
            }
          },
         "sort": [
           {
              "timeStamp": "desc"
           }
        ]
     }`
    queryResults, err := getQueryResultForQueryString(stub, queryString)
    if err != nil {
        return shim.Error(err.Error())
    }
    return shim.Success(queryResults)

Here is the index :

{
    "index":{
        "fields":["dockType","timeStamp"] 
    },
    "ddoc":"indexDoc", 
    "name":"index1Doc",
    "type":"json"
  }

There is the index document too in my couchdb fauxton.

fauxton UI

But it is giving the error like :

Error: GET_QUERY_RESULT failed: transaction ID: 734c4e0c48be97ee176bb597acf7bf0cba03c0c06954200c84a78437b75afd94: error handling CouchDB request. Error:no_usable_index, Status Code:400, Reason:No index exists for this sort, try indexing by the sort fields.