title: exception: "mutating method sent to immutable object" which is actually a mutable array
body: <p>I am trying to change the value to a specific key for a mutable array:</p>
  
  <pre><code>NSMutableArray* reversed = [NSMutableArray arrayWithArray:regions];
  
  [reversed setValue:@"current" forKey:@"region"];
  </code></pre>
  
  <p>But this leads to an uncaught exception:</p>
  
  <blockquote>
    <p><strong>* Terminating app due to uncaught exception
    'NSInternalInconsistencyException', reason: '-[__NSCFDictionary
    setObject:forKey:]: mutating method sent to immutable object'
    *</strong> First throw call stack:</p>
  </blockquote>
  
  <p>Why is it telling me that my NSMutableArray is an immutable object?</p>