# An exception caught while applying mutate filter - Could not set field 'log' on object '' to value '...'

**URL:** https://discuss.elastic.co/t/an-exception-caught-while-applying-mutate-filter-could-not-set-field-log-on-object-to-value/244583
**Category:** Logstash
**Created:** [August 11, 2020, 3:19pm UTC](https://discuss.elastic.co/t/an-exception-caught-while-applying-mutate-filter-could-not-set-field-log-on-object-to-value/244583 "2020-08-11T15:19:21Z")
**Posts on this page:** 1
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### Author: ![Badger](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/badger/32/25190_2.png) [@Badger](https://discuss.elastic.co/u/Badger)
#### Post date: [August 11, 2020, 3:37pm UTC](https://discuss.elastic.co/t/an-exception-caught-while-applying-mutate-filter-could-not-set-field-log-on-object-to-value/244583/2 "2020-08-11T15:37:47Z")

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Generally that exception is trying to tell you that your destination field is a concrete value, and you are trying to make it an object. The exception can be [misleading](https://discuss.elastic.co/t/logstash-crashes-when-converting-fields-to-ecs-format/165306/2) because

1. "on object ''" prints the value of the field, not the name of the field
2. "is not either a map or a string" is just wrong, because the exception gets thrown when the field is a string!

So ... [metadata] exists, but is a value, not an object. Could it be a boolean?

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_[View the full topic](https://discuss.elastic.co/t/an-exception-caught-while-applying-mutate-filter-could-not-set-field-log-on-object-to-value/244583)._