title: MongoDB index is not being used
body: <p>I have a collection of questions with index on modified.</p>
  
  <pre><code>{
    "v" : 1,
    "key" : {
        "modified" : 1
    },
    "name" : "modified_1",
    "ns" : "app23568387.questions",
    "background" : true,
    "safe" : null
  }
  </code></pre>
  
  <p>But when I query the questions with modified field, mongo does not use this index.</p>
  
  <pre><code>db.questions.find({modified: ISODate("2016-07-20T20:58:20.662Z")}).explain(true);
  </code></pre>
  
  <p>It returns</p>
  
  <pre><code>{
    "cursor" : "BasicCursor",
    "isMultiKey" : false,
    "n" : 0,
    "nscannedObjects" : 19315626,
    "nscanned" : 19315626,
    "nscannedObjectsAllPlans" : 19315626,
    "nscannedAllPlans" : 19315626,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 384889,
    "nChunkSkips" : 0,
    "millis" : 43334,
    "allPlans" : [ 
        {
            "cursor" : "BasicCursor",
            "isMultiKey" : false,
            "n" : 0,
            "nscannedObjects" : 19315626,
            "nscanned" : 19315626,
            "scanAndOrder" : false,
            "indexOnly" : false,
            "nChunkSkips" : 0
        }
    ],
    "server" : "c387.candidate.37:10387",
    "filterSet" : false,
    "stats" : {
        "type" : "COLLSCAN",
        "works" : 19624020,
        "yields" : 384889,
        "unyields" : 384889,
        "invalidates" : 3,
        "advanced" : 0,
        "needTime" : 19315627,
        "needFetch" : 0,
        "isEOF" : 1,
        "docsTested" : 19315626,
        "children" : []
    }
  }
  </code></pre>
  
  <p>When I use <code>hint()</code>, mongo throws an error <code>bad hint</code>.
  I have another collection of folders which has exactly the same index and the query uses the index. (returns <code>"cursor" : "BtreeCursor modified_1"</code> for <code>explain()</code>)</p>
  
  <p>What could be the difference between questions and folders? Is it possible that the index is "broken" even though <code>getIndexes()</code> returns it? If so, what can I do to fix it?</p>