title: -[__NSCFDictionary setObject:forKey:]: mutating method sent to immutable object
body: <p>When running the below code, the <code>[dict setValue:@"null" forKey:@"name"];</code> keeps crashing. I search on here and found other posts were caused by people not using <code>NSMutableDictionary</code>. However I am using this.</p>
  
  <p>Why is it crashing on this line if <code>name</code> is <code>null</code>?</p>
  
  <pre><code>NSMutableArray *tempCustomers = [[NSMutableArray alloc] init];
  for (NSMutableDictionary *dict in [[json objectForKey:@"data"] mutableCopy]) {
      if ([dict objectForKey:@"name"] == [NSNull null]) {
          [dict setValue:@"null" forKey:@"name"];
      }
      [tempCustomers addObject:dict];
  }
  </code></pre>