Jenkins logstash plugin data.buildDuration always 0

I installed the plugin logstash for jenkins, and while trying to send logs on ElasticSearch in order to display them on kibana. Everything works except for the data buildDuration field which is always equal to 0.

    {
  "_index": "logstash-2014.11.12",
  "_type": "logs",
  "_id": "tmeu84-rToWJ_8N6PuHBzg",
  "_score": null,
  "_source": {
    "data": {
      "id": "2014-11-12_13-59-37",
      "result": "SUCCESS",
      "projectName": "testredmine",
      "displayName": "#26",
      "fullDisplayName": "testredmine #26",
      "url": "job/testredmine/26/",
      "buildHost": "Jenkins",
      "buildLabel": "",
      "buildNum": 26,
      "buildDuration": 0,
      "timestamp": "2014-11-12T13:59:37+0100",
      "rootProjectName": "testredmine",
      "rootProjectDisplayName": "#26",
      "rootBuildNum": 26,
      "buildVariables": {}
    },
    "message": [
      "[EnvInject] - Loading node environment variables.",
      "Building on master in workspace /home/jenkins/data/jobs/testredmine/workspace",
      "FSSCM.checkout /tmp/testlogstash/ to /home/jenkins/data/jobs/testredmine/workspace",
      "FSSCM.check completed in 19 milliseconds",
      "Hello, null!",
      "[workspace] $ /bin/sh -xe /tmp/hudson7203903808744352734.sh",
      "+ sleep 5"
    ],
    "source": "jenkins",
    "source_host": "http://jenkins-test/",
    "@timestamp": "2014-11-12T13:59:37.000+01:00",
    "version": 1,
    "@version": "1"
  },
  "sort": [
    1415797177000,
    1415797177000
  ]
}

I am new to JENKINS and lost in this. Please help me.