# Documentation error
**URL:** https://discuss.elastic.co/t/documentation-error/253330
**Category:** Logstash
**Created:** [October 26, 2020, 2:57pm UTC](https://discuss.elastic.co/t/documentation-error/253330 "2020-10-26T14:57:31Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@gborg](https://discuss.elastic.co/u/gborg)
#### Post date: [October 26, 2020, 2:57pm UTC](https://discuss.elastic.co/t/documentation-error/253330/1 "2020-10-26T14:57:31Z")
In the logstash mutete filter documentation there seems to be an error, or at least it seems like an error to me
```auto
mutate {
split => ["hostname", "."]
add_field => { "shortHostname" => "%{hostname[0]}" }
}
```
does not work, however this works
```auto
mutate {
split => ["hostname", "."]
add_field => { "shortHostname" => "%{hostname][0]}" }
}
```
Both the field name and the indexing number need to be in square brackets
> **[Mutate filter plugin | Logstash Reference \[7.9\] | Elastic](https://www.elastic.co/guide/en/logstash/current/plugins-filters-mutate.html#plugins-filters-mutate-add_tag)**
---
### Author:  [@Badger](https://discuss.elastic.co/u/Badger)
#### Post date: [October 26, 2020, 4:39pm UTC](https://discuss.elastic.co/t/documentation-error/253330/2 "2020-10-26T16:39:01Z")
> [@gborg](#):
>
> it seems like an error to me
Indeed. "%{hostname[0]}" worked up until the 7.0 release, at which point it started to result in an error.
---
### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [November 23, 2020, 4:39pm UTC](https://discuss.elastic.co/t/documentation-error/253330/3 "2020-11-23T16:39:12Z")
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