title: 'NSString' is not convertible to 'DictionaryIndex<NSObject, AnyObject>'
body: <p>I try to rewrite some objective c code into swift. Now I get a very strange error. I tried the code in a playgroud, absolutely untouched from other code.</p>
  
  <p>Here is the part, I want to translate:</p>
  
  <pre><code>NSDictionary *info = [self infoForBinding:@"theBinding"];
  [[info objectForKey:NSObservedObjectKey] 
          setValue:MyValue 
          forKeyPath:[info objectForKey:NSObservedKeyPathKey]];
  </code></pre>
  
  <p>This is the code, I tried:</p>
  
  <pre><code>class x: NSButtonCell {
  
      func a() {
          var info = infoForBinding("theBinding")
          info[NSObservedObjectKey]?.setValue(nil, forKeyPath: info[NSObservedKeyPathKey])
      }
  
  }
  </code></pre>
  
  <p>Now I get the error <code>'NSString' is not convertible to 'DictionaryIndex&lt;NSObject, AnyObject&gt;'</code>. Any ideas?</p>
  
  <p>UPDATE:</p>
  
  <p>This is working... I don't know why.</p>
  
  <pre><code>info[NSObservedObjectKey]?.setValue(MyValue forKeyPath: (info[NSObservedKeyPathKey] as NSString))
  </code></pre>