title: Cast from 'JSON' to unrelated type 'String' always fails
body: <p>Currently I'm using <code>SwiftyJSON</code> to convert the data set into JSON type.</p>
  
  <p>When I run this: <code>print(json)</code>, it will print on the Xcode console.</p>
  
  <pre><code>{
    "user_id": "123999923821" 
  }
  </code></pre>
  
  <p>But whenever I try to do execute <code>json["user_id"] as? String</code>, it always returns nil.</p>
  
  <pre><code>socketManager.socket.on("toAllDrivers") { data, ack in
              if let vc = self.storyboard?.instantiateViewController(withIdentifier: "PopViewControllerID") as? PopUpViewController {
  
                  let json = JSON(data[0])
                  print(json)
                  vc.passenger_id = json["user_id"] as? String // This is the line of code that return a problem.
  
                  self.present(vc, animated: true, completion: nil)
              }
  
  
          }
  </code></pre>
  
  <p><code>vc.passenger_id</code> is a type String:</p>
  
  <pre><code>var passenger_id: String?
  </code></pre>
  
  <p>The error is:</p>
  
  <blockquote>
    <p>Cast from 'JSON' to unrelated type 'String' always fails</p>
  </blockquote>
  
  <p>What should I do to fix this?</p>