title: "There is no index available for this selector" despite the fact I made one
body: <p>In my data, I have two fields that I want to use as an index together. They are <code>sensorid</code> (any string) and <code>timestamp</code> (yyyy-mm-dd hh:mm:ss).</p>
  
  <p>So I made an index for these two using the Cloudant index generator. This was created successfully and it appears as a design document.</p>
  
  <pre><code>{
    "index": {
      "fields": [
        {
      "name": "sensorid",
      "type": "string"
        },
        {
      "name": "timestamp",
      "type": "string"
        }
      ]
    },
    "type": "text"
  }
  </code></pre>
  
  <p>However, when I try to make the following query to find all documents with a timestamp newer than some value, I am told there is no index available for the selector:</p>
  
  <pre><code>{
    "selector": {
      "timestamp": {
        "$gt": "2015-10-13 16:00:00"
      }
    },
    "fields": [
      "_id",
      "_rev"
    ],
    "sort": [
      {
        "_id": "asc"
      }
    ]
  }
  </code></pre>
  
  <p>What have I done wrong?</p>