title: Elastalert is not alerting with Slack channel
body: <p>I am setting up the Elastalert alert system with Slack.</p>
  <p>Here is the Elastalert config.</p>
  <pre><code>s_host: localhost
  es_port: 9200
  name: Slack rule
  type: frequency
  index: filebeat-*
  num_events: 1
  timeframe:
    minutes: 1
  filter:
  - term:
      msg: &quot;proper name is required&quot;
  alert:
  - &quot;slack&quot;
  slack_webhook_url: &quot;https://hooks.slack.com/services/SSDFRD/SDDDDS/XXXSDDDSDDDDDDDSS&quot;
  slack_channel_override: &quot;#kibana&quot;
  slack_username_override: &quot;@anand&quot;
  </code></pre>
  <p>Getting below error</p>
  <pre><code>ERROR:root:Error finding recent pending alerts: RequestError(400, 'search_phase_execution_exception', 'No mapping found for [alert_time] in order to sort on') 
  
  {
     &quot;query&quot;:{
        &quot;bool&quot;:{
           &quot;must&quot;:{
              &quot;query_string&quot;:{
                 &quot;query&quot;:&quot;!_exists_:aggregate_id AND alert_sent:false&quot;
              }
           },
           &quot;filter&quot;:{
              &quot;range&quot;:{
                 &quot;alert_time&quot;:{
                    &quot;from&quot;:&quot;2020-08-25T13:33:31.764880Z&quot;,
                    &quot;to&quot;:&quot;2020-08-27T13:33:31.764901Z&quot;
                 }
              }
           }
        }
     },
     &quot;sort&quot;:{
        &quot;alert_time&quot;:{
           &quot;order&quot;:&quot;asc&quot;
        }
     }
  }
  </code></pre>
  <p>Elasticsearch record</p>
  <pre><code>{
  _index: &quot;filebeat-log&quot;,
  _type: &quot;doc&quot;,
  _id: &quot;m3DVLnQBAkDXq2X4hDio&quot;,
  _score: 1,
  _source: {
  time: &quot;2020-08-27T07:32:58.075Z&quot;,
  msg: &quot;proper name is required&quot;,
  name: &quot;myApp&quot;,
  v: 0,
  @timestamp: &quot;2020-08-27T07:32:59.048Z&quot;,
  host: &quot;AMM00361&quot;,
  pid: 5026,
  level: 50,
  hostname: &quot;AMM00361&quot;,
  type: &quot;myApp&quot;,
  @version: &quot;1&quot;,
  path: &quot;/var/log/myapp-error.log&quot;
  }
  }
  </code></pre>
  <p>After running the Elastalert service I couldn't get the alert message in Slack.</p>
  <p>Please share your thoughts!</p>