title: Exception: can't convert from BSON type EOO to Date
body: <p>I am getting an issue for running the following aggregate query:</p>
  <pre><code>db.snippets.aggregate([ { '$project': { month: { '$month': '$created_at' }} } ])
  </code></pre>
  <p>The error message for the same is:</p>
  <pre><code>assert: command failed: {
          &quot;errmsg&quot; : &quot;exception: can't convert from BSON type EOO to Date&quot;,
          &quot;code&quot; : 16006,
          &quot;ok&quot; : 0 } : aggregate failed
  </code></pre>
  <p>How do I get around this issue? I found a related question: <a href="https://stackoverflow.com/questions/16194909/mongodb-cant-convert-from-bson-type-eoo-to-date">MongoDB: can&#39;t convert from BSON type EOO to Date</a>.</p>
  <p>But it doesn't tell me how to get things done.</p>