title: Attempt to insert nil
body: <p>Seems like it should be easy to add a boolean to an NSMutableArray.</p>
  
  <p>Assume toDoArray is intialized as an NSMutableArray. The following:</p>
  
  <pre><code>BOOL checkBoxState = NO;
  [toDoArray addObject:checkBoxState]; 
  </code></pre>
  
  <p>Generates the error "attempt to insert nil."</p>
  
  <p>What's the correct way to add a negative boolean to a mutable array?</p>