Logstash errors while executing Ruby Code

Elastic Stack Logstash
1

Hi :
I am trying to calculate the difference between two timestamp fields to see if we have an issue in consuming the data. (the @timestamp field automagically created to indicate index time and date field generated by converting a date String field. )

this is the config :

input {
   kafka {
      zk_connect => "zkblah:2181"
      consumer_threads => 1
      topic_id => "blah_topic"
      group_id => "blah_group"
   }
}
filter {
  date {
     match => [ "created_at","EEE MMM dd HH:mm:ss Z yyyy" ]
     target => "converted_created_at"
  }
  ruby {
    code => "event['diff_time'] = Time.parse(event['@timestamp'] - event['converted_created_at']).to_f "
  }
}

output {
   elasticsearch {
     hosts => "host:9200"
     index => "mutate_follow_filter-%{+YYYY.MM.dd}"
     document_type => "blahblah"
     flush_size => 1000
     workers => 4
   }
# stdout {
#    codec => "dots"
#  }
}

When I run the logstash process, I get the following error :
Ruby exception occurred: allocator undefined for Float {:level=>:error}
Ruby exception occurred: allocator undefined for Float {:level=>:error}
Ruby exception occurred: allocator undefined for Float {:level=>:error}
Ruby exception occurred: allocator undefined for Float {:level=>:error}
Ruby exception occurred: allocator undefined for Float {:level=>:error}
Ruby exception occurred: allocator undefined for Float {:level=>:error}

is there some magic that I am missing... I am basically trying to do some simple date math and its throwing logstash for a loop.

any and all help is appreciated.

(using logstash 2.2.2, Java 8 and ES 2.1.1)

Thanks

Ramdev