Mark "target" field as required in json filter

Without it in config file logstash is throwing following error for completely valid JSON file:
:exception=>#<NoMethodError: undefined method `[]' for nil:NilClass>, :level=>:warn

Steps to reproduce:
echo '{ "attr": "val"}' | java -jar logstash-1.1.13-flatjar.jar agent -f logstash-simple.conf

logstash-simple.conf
<pre><code>
input { 
    stdin { type => "stdin-type"}
}

filter {
  json {
    source => "@message"
  }
}

output { stdout { debug => true debug_format => "json"}}
</code></pre>