title: $add operation returns null value
body: <p>I have collection with document structure : </p>
  
  <pre><code> {
    'year' : 2014,
    'month' : 1
   }
  </code></pre>
  
  <p>I am executing the following operation : </p>
  
  <pre><code>db.collname.aggregate(
                          [
                             {
                               $project : {
                                      'year100' : {$multiply : ["$year" , 100]},
                                      'result' : { '$add' : ['$year100', '$month'] }       
                                          }
                             }
                          ]
                        );
  </code></pre>
  
  <p>I get the following result : </p>
  
  <pre><code> {
  "result" : [ 
      {
          "_id" : ObjectId("5563596c515a88832210f0e4"),
          "year100" : 201400.0000000000000000,
          "result" : null
      },
  }
  </code></pre>
  
  <p>Why is add operation returuning null value as against to actual value ? Please help.</p>