# ClassCastException\[null\] when grouping on a particular field
**URL:** https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604
**Category:** Elasticsearch
**Created:** [February 1, 2016, 6:29am UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604 "2016-02-01T06:29:28Z")
**Posts on this page:** 11
**Page:** 1
### Author:  [@Ankur\_Kumar](https://discuss.elastic.co/u/Ankur_Kumar)
#### Post date: [February 1, 2016, 6:29am UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604/1 "2016-02-01T06:29:29Z")
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?
---
### Author:  [@Geunmoon\_Oh](https://discuss.elastic.co/u/Geunmoon_Oh)
#### Post date: [February 1, 2016, 6:51am UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604/2 "2016-02-01T06:51:56Z")
to solve this, mapping document is required.
---
### Author:  [@dadoonet](https://discuss.elastic.co/u/dadoonet)
#### Post date: [February 1, 2016, 7:01am UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604/3 "2016-02-01T07:01:42Z")
Are you running that accross multiple indices?
Could be a mapping issue.
Are you sure you don't have a full stack trace?
Which version?
---
### Author:  [@Ankur\_Kumar](https://discuss.elastic.co/u/Ankur_Kumar)
#### Post date: [February 1, 2016, 7:20am UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604/4 "2016-02-01T07:20:18Z")
- No, I am not running it across multiple indices.
- Yes, I did not have a full stack trace for that particular exception. I faced a new exception after posting this question and the trace is as below:
`[2016-02-01 12:42:28,773][DEBUG][action.search.type] [elastic71] failed to reduce search org.elasticsearch.action.search.ReduceSearchPhaseException: Failed to execute phase [fetch], [reduce] at org.elasticsearch.action.search.type.TransportSearchQueryThenFetchAction$AsyncAction$2.onFailure(TransportSearchQueryThenFetchAction.java:159) at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:41) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at java.lang.Thread.run(Thread.java:745) Caused by: java.lang.ClassCastException`
- The version is 1.7.3
---
### Author:  [@Ankur\_Kumar](https://discuss.elastic.co/u/Ankur_Kumar)
#### Post date: [February 1, 2016, 7:29am UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604/5 "2016-02-01T07:29:44Z")
> **[Mapping-Document](https://docs.google.com/document/d/1Tvo23nfXuvZIm9_LT5ITxi59BPxpV7F4EsUdu8CLZUQ/edit?usp=sharing)**
>
> The mapping document is : \`{ "trafficdata\_sessions\_3": { "mappings": { "session": { "dynamic\_templates": \[ { "en\_session": { "mapping": { "type": "string", ...
---
### Author:  [@dadoonet](https://discuss.elastic.co/u/dadoonet)
#### Post date: [February 1, 2016, 7:32am UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604/6 "2016-02-01T07:32:45Z")
Upgrade if possible. IIRC lot of things have been fixed since then.
---
### Author:  [@Geunmoon\_Oh](https://discuss.elastic.co/u/Geunmoon_Oh)
#### Post date: [February 1, 2016, 7:34am UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604/7 "2016-02-01T07:34:35Z")
i guess some fields are conflicted.
so index mapping information is needed.
---
### Author:  [@Ankur\_Kumar](https://discuss.elastic.co/u/Ankur_Kumar)
#### Post date: [February 1, 2016, 7:39am UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604/8 "2016-02-01T07:39:10Z")
Please find the document mapping [here](https://docs.google.com/document/d/1Tvo23nfXuvZIm9_LT5ITxi59BPxpV7F4EsUdu8CLZUQ/edit?usp=sharing):
---
### Author:  [@Geunmoon\_Oh](https://discuss.elastic.co/u/Geunmoon_Oh)
#### Post date: [February 1, 2016, 8:20am UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604/9 "2016-02-01T08:20:42Z")
i received it.
i think mapping information is correct.
and i tested it (ES 1.7.2 and 2.1). but no error..
---
### Author:  [@Ankur\_Kumar](https://discuss.elastic.co/u/Ankur_Kumar)
#### Post date: [February 1, 2016, 1:07pm UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604/10 "2016-02-01T13:07:52Z")
I got the instance restarted and this fixed the issue.
I assume that it was some corrupt cache / index internally which got cleaned up once the instance was restarted.
---
### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [July 5, 2017, 11:19pm UTC](https://discuss.elastic.co/t/classcastexception-null-when-grouping-on-a-particular-field/40604/11 "2017-07-05T23:19:54Z")