title: doesnotrecognizeselector exception
body: <p>Using NSXMLParser</p>
  
  <p>why does this line of code cause an exception:</p>
  
  <pre><code>ushort crash = [[attributeDict objectForKey:@"a"] unsignedShortValue];
  </code></pre>
  
  <p>The value in the xml file is "0"</p>
  
  <p>if I use integerValue it works</p>
  
  <p>what is happening?</p>