Error:Class cast exception in elastic search while sorting buckets in aggregation

Question

Error: ClassCastException[org.elasticsearch.search. aggregations.support.ValuesSource$Bytes$WithOrdinals$FieldData cannot be cast to org.elasticsearch.search.aggregations.support.ValuesSource$Numeric]}{[vTHdFzpuTEGMGR8MES_b9g]

My Query:

         GET _search
{
  "size" : 0,
  "query" : {
    "filtered" : {
      "query" : {
        "dis_max" : {
          "tie_breaker" : 0.7,
          "queries" : [ {
            "bool" : {
              "should" : [ {
                "match" : {
                  "post.body" : {
                    "query" : "check",
                    "type" : "boolean"
                  }
                }
              }, {
                "match" : {
                  "post.parentBody" : {
                    "query" : "check",
                    "type" : "boolean",
                    "boost" : 2.0
                  }
                }
              } ]
            }
          } ]
        }
        }
        }
        },
  "aggregations" : {
    "by_parent_id" : {
      "terms" : {
        "field" : "post.parentId",
        "order" : {
          "max_score" : "desc"
        }
      },
      "aggregations" : {
        "max_score" : {
          "max" : {}
        },
        "top_post" : {
          "top_hits" : {
            "size" : 1
          }
          }
        }
    }
  }

I want to sort buckets by max_score rather than by doc_count which is the default behaviour of elastic search. I am trying to aggregate posts (which contains body and parentBody) by parentId and then sorting buckets by max_score and in each bucket I am getting top_hits. But I am getting the above error when I sorted the buckets by defining max score aggregation. Rest everything works if I remove max_score aggregation. Every post object has parentId, body and parentBody. I have used the following references for coding this:

Elasticsearch Aggregation: How to Sort Bucket Order https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-top-hits-aggregation.html#_field_collapse_example

Tell me what am I doing wrong? I have shared the query above.