title: Logstash Array split gives Nilclass for one element array
body: <p>I am trying to input json data from logs through filebeat-> logstash  to elasticsearch but i seem to get NilClass error no matter what i try.</p>
  
  <p>The data sample:</p>
  
  <pre><code>{"student":[{"details":{"name":chirs,"lname":dave},"age":10,"grade":1.2,"id":"323"}],"id":"metric95"}
  </code></pre>
  
  <p>my logstash configuration is:</p>
  
  <pre><code>input {
      beats {
          port =&gt; "5044"
      }
  }
  filter {
  json {
      source =&gt; "message"
    }
   split {
        field =&gt; "[student]"
      }
  }
  output {
  elasticsearch {
          hosts =&gt; [ "localhost:9200" ]
      }
  
      stdout { codec =&gt; rubydebug }
  } 
  
  Error:  split - Only String and Array types are splittable. field:[student] is of type = NilClass
  </code></pre>