# Logstash substring a field using position getting error
**URL:** https://discuss.elastic.co/t/logstash-substring-a-field-using-position-getting-error/292157
**Category:** Logstash
**Created:** [December 16, 2021, 1:21pm UTC](https://discuss.elastic.co/t/logstash-substring-a-field-using-position-getting-error/292157 "2021-12-16T13:21:36Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@aeoker](https://discuss.elastic.co/u/aeoker)
#### Post date: [December 16, 2021, 1:21pm UTC](https://discuss.elastic.co/t/logstash-substring-a-field-using-position-getting-error/292157/1 "2021-12-16T13:21:37Z")
Hello Everyone,
I am getting below error while trying to produce new event from existing one as follows, any help?
```
ruby { code => "event['new_field'] = event.get['dummy'][1..4]" }
```
Error:
> Ruby exception occurred: undefined method `[]' for nil:NilClass {:class=>"NoMethodError", :backtrace=>["(ruby filter code):2:in `block in filter\_method'", "./LOGSTASH/logstash-7.16.1/vendor/bundle/jruby/2.5.0/gems/logstash-filter-ruby-3.1.7/lib/logstash/filters/ruby.rb:93:in `inline_script'", "./LOGSTASH/logstash-7.16.1/vendor/bundle/jruby/2.5.0/gems/logstash-filter-ruby-3.1.7/lib/logstash/filters/ruby.rb:86:in `filter'", "./LOGSTASH/logstash-7.16.1/logstash-core/lib/logstash/filters/base.rb:159:in `do_filter'", "./LOGSTASH/logstash-7.16.1/logstash-core/lib/logstash/filters/base.rb:178:in `block in multi\_filter'", "org/jruby/RubyArray.java:1821:in `each'", "/evam/LOGSTASH/logstash-7.16.1/logstash-core/lib/logstash/filters/base.rb:175:in `multi\_filter'", "org/logstash/config/ir/compiler/AbstractFilterDelegatorExt.java:134:in `multi_filter'", "./LOGSTASH/logstash-7.16.1/logstash-core/lib/logstash/java_pipeline.rb:299:in `block in start\_workers'"]}
sample conf:
```auto
input {
stdin { id => "inPut" }
}
filter{
ruby { code => "event['new_field'] = event.get['dummy'][1..4]" }
}
output {
stdout { codec => rubydebug }
}
```
---
### Author:  [@Badger](https://discuss.elastic.co/u/Badger)
#### Post date: [December 16, 2021, 4:10pm UTC](https://discuss.elastic.co/t/logstash-substring-a-field-using-position-getting-error/292157/2 "2021-12-16T16:10:13Z")
> [@aeoker](#):
>
> undefined method `[]' for nil:NilClass`
That is telling you that the [dummy] field does not exist, so event.get returns nil, and an attempt to extract a substring of nil fails.
---
### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [January 13, 2022, 4:10pm UTC](https://discuss.elastic.co/t/logstash-substring-a-field-using-position-getting-error/292157/3 "2022-01-13T16:10:32Z")
This topic was automatically closed 28 days after the last reply. New replies are no longer allowed.