title: Unknown key for a VALUE_STRING in [search_after] error
body: <p>I am making an elastic search call for a result set of 18,000 subjects. Thus I am using search_after. But I am getting following error after the very first use of this parameter.</p>
  
  <pre><code>Unknown key for a VALUE_STRING in [search_after]
  </code></pre>
  
  <p>I am using elastic search 5.1 , and the query is as follows </p>
  
  <pre><code>{
    "query": {
      "bool": {
        "filter": [
          {
            "bool": {
              "should": [
                {
                  "match": {
                    "CategoryId": "1-4534"
                  }
                }
              ],
              "minimum_should_match": 1
            }
          }
        ]
      }
    },
    "size": 100,
    "from": 0,
    "sort": {"Name":"asc"} ,
    "search_after": "D106891_1"
  }
  
  </code></pre>
  
  <p>Requirement is not to set any explicit limit on number of results that can be returned by elastic search (which defaults to 10,000) but some pagination that keeps the code free from any issue in case user data crosses a set limit in future. I am performing pagination by sending the query in a loop, getting 100 subjects in one go. This is accomplished by setting size to 100 and increment value of 
   <em>from</em>  by 100 each iteration. When I reach  <em>from+size = 10000</em> , I am adding  <em>search_after</em>  but it fails with the error mentioned above. Please let me know if I can provide with any more info. Thanks.</p>