在iOS中使用Swift更改UIAlertAtroller的UIAlertController按钮标题的字体大小时出错

Question

let alertController: UIAlertController = UIAlertController(title: "Alert", message: nil, preferredStyle: .ActionSheet)

let customActionButton: UIAlertAction = UIAlertAction(title: "Custom", style: .Default) { action -> Void in

}
let attrString: NSAttributedString = NSAttributedString(string: "Custom", attributes: [NSFontAttributeName: UIFont.init(name: "Roboto-Regular", size: 17.0)!])
customActionButton.setValue(attrString, forKey: "attributedTitle")
alertController.addAction(customActionButton)

self.presentViewController(alertController, animated: true, completion: nil)

我收到以下错误：

  

＆＃39; [setValue：forUndefinedKey：]：此类与key _attributedTitle不符合键值编码。＆＃39;

请告诉我更改UIAlertAction按钮标题字体大小的正确键值。

任何帮助都表示赞赏.. !!