title: Type '[String, AnyObject?]' does not conform to protocol AnyObject?: why?
body: <p>I'm trying to understand why I have a compilation error: <strong>Type '[String, AnyObject?]' does not conform to protocol AnyObject?</strong></p>
  
  <pre><code>var cars: [String: AnyObject?] = [ "model": "Ferrari"]
  var JSON: [String: AnyObject?] = [ "cars": cars ]  // &lt;-- error
  </code></pre>
  
  <p>It seems like <code>cars</code> is not an AnyObject? which I don't understand.</p>
  
  <p>Thanks for your help!</p>