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

Elastic Stack Logstash
2

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 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?