title: NSMutableDictionary setObject:forKey did't work when value is NSMutableArray
body: <p>dict is NSMutableDictionary; array is NSMutableArray and is NOT nil;</p>
  
  <p>The code:</p>
  
  <pre><code>[dict setObject:array forKey:@"key"];
  </code></pre>
  
  <p>There is no error or warning but the dict is null,nothing is set or added.</p>
  
  <p>When I use the code below,it works:</p>
  
  <pre><code>[dict setObject:[NSArray arrayWithArray:array] forKey:@"Key"];
  </code></pre>
  
  <p>Can anyone tell me why ?</p>
  
  <p><strong>Update:</strong>
  Both <code>dict</code> and <code>array</code> are local variables and have been initialized.The <code>dict</code> is nil.</p>
  
  <p><code>NSLog(@"%@", array)</code> has printed the value of array:</p>
  
  <pre><code>({"Title":"firstTitle","Date":"20110101"},{"Title":"secondTitle","Date":"20110102"})
  </code></pre>
  
  <hr>
  
  <h2>UPDATE:</h2>
  
  <p>I have made a mistake.The <code>array</code> is not null at first,but I emptied it in follow operation.</p>
  
  <p>Thanks for @Bavarious.</p>