Count by query with es-5.0.0-RC1 parsing exception

We have an Issue with the current Elasticsearch version
POST https://estest.dev1.ehm.rocks:443/testrest/test_type/_count: HTTP/1.1 400 Bad Request
{"error":{"root_cause":[{"type":"parsing_exception","reason":"request does not support [ext]","line":7,"col":3}],"type":"parsing_exception","reason":"request does not support [ext]","line":7,"col":3},"status":400}`

We use the elasticsearch Restclient and the SearchSourceBuilder

SearchSourceBuilder ssb = new SearchSourceBuilder();
ssb.query(QueryBuilders.matchAllQuery());
CountResponse countByQuery = testClient.countByQuery(ssb);

The ssb seems to have the ext key and elasticsearch cannot handle it

{
  "query" : {
    "match_all" : {
      "boost" : 1.0
    }
  },
  "ext" : { }
}

Our Restclient:

public CountResponse countByQuery(SearchSourceBuilder ssb) throws IOException
    {
        CountResponse result = null;
        Response response = restClient.performRequest("POST",
                "/" + index + "/" + indexType + "/_count", new Hashtable<>(), stringentity(ssb.toString().getBytes(UTF_8)),
                getHeaders());
        result = JsonMapping.stringToObject(responseContentToString(response), CountResponse.class);

        return result;
    }

I am facing the same issue. Did you figure out what was the problem ?