title: iOS: "attempt to insert nil object from objects[1]" when creating dictionary
body: <p>I'm creating a custom navigation bar class and customizing it's title attributes using the following code:</p>
  
  <pre><code>self.titleTextAttributes = @{ UITextAttributeFont: bariol,
                                UITextAttributeTextColor: [UIColor whiteColor]
                              };
  </code></pre>
  
  <p>However, when I run the code, it returns the following error message:</p>
  
  <pre><code>*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[1]'
  </code></pre>
  
  <p>I'm using Xcode 4.6 and running on a device using iOS 6.</p>