title: NSPropertyListSerialization returning nil instead of serialized data
body: <p>I am putting two <code>NSMutableArray</code> objects into an <code>NSDictionary</code> and trying to serialize, but the method call is returning nil. One array, <code>addresses</code>, is an array of <code>NSString</code> objects. The other, <code>engines</code> is an array of objects that each contain several data types. I am attempting to serialize using the following code:</p>
  
  <pre><code>NSMutableDictionary *dictionary = [[NSMutableDictionary alloc] init];
  [dictionary setObject:engAddr forKey:@"engAddr"];
  [dictionary setObject:trainList forKey:@"engines"];
  NSData *data = [NSPropertyListSerialization dataFromPropertyList:dictionary
                                                            format:NSPropertyListXMLFormat_v1_0
                                                  errorDescription:&amp;error];
  </code></pre>
  
  <p>Stepping through, the debugger shows the arrays are properly added to the dictionary, but after the line that should serialize the dictionary it shows <code>data = (NSData *) nil</code>.</p>
  
  <p>Where am I going wrong? Thank you for your help!</p>