NSCoreData - Textfield为浮点值

Question

当用户按下提交按钮时，我试图将我的文本字段添加到coredata中作为浮点值。我有一个错误

  

线程1：EXC_BAD_ACCESS（代码= 1，地址= 0x1）

我在.xcdatamodel文件和NSManagedObject类中的属性设置为float类型。

 @IBOutlet weak var forceEntered: UITextField!
 @IBOutlet weak var stiffnessEntered: UITextField!


 @IBAction func submit(sender: AnyObject) {

        let appDel : AppDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
        let contxt : NSManagedObjectContext = appDel.managedObjectContext!
        let en = NSEntityDescription.entityForName("SpringVariables", inManagedObjectContext: contxt)
        var newItem = SpringModel(entity:en!,insertIntoManagedObjectContext: contxt)

        newItem.stiffness = (stiffnessEntered.text as NSString).floatValue //error message shows here.
        newItem.force = (forceEntered.text as NSString).floatValue


 }