title: App Crashing: Mutating method sent to immutable object
body: <p>I am trying to add an object to an <code>NSMutableArray</code>. Initially I assign some response data to the array, and can display it in a table view. After loading more data, it seems to be crashing when trying to add the new information to my original array. </p>
  
  <p>I am using AFNetworking for this: </p>
  
  <pre><code>[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
  
      if(!_myArray){
          _myArray = [responseObject objectForKey:@"data"];
      }
      else{
          [_myArray addObject:[responseObject objectForKey:@"data"]];
      }
      [self.tableView reloadData];
  }
  </code></pre>
  
  <p>The error I am getting is as follows </p>
  
  <pre><code>*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', 
  reason: '-[__NSCFArray insertObject:atIndex:]: mutating method sent to immutable object'
  </code></pre>
  
  <p>Can anybody help out with this?</p>