title: Cannot invoke 'append' with an argument list of type '(String?!)'
body: <p>I'm trying to add usernames from Parse in an array to display them in a UITableView, but get an error when I'm appending the usernames to my array.</p>
  
  <p>The error I get is: Cannot invoke 'append' with an argument list of type '(String?!)'</p>
  
  <p>What am I doing wrong?</p>
  
  <pre><code>var usernames = [""]
  
  func updateUsers() {      
      var query = PFUser.query()
      query!.whereKey("username", notEqualTo: PFUser.currentUser()!.username!)
      var fetchedUsers = query!.findObjects()
  
      for fetchedUser in fetchedUsers! {
          self.usernames.append(fetchedUser.username)
      }
  }
  </code></pre>