Thread 1: EXC_BAD_INSTRUCTIONS | SWIFT

When i am starting the app, it runs normal. When i click randomize button, "Thread 1: EXC_BAD_INSTRUCTIONS" comes at "finalLABEL.text? = str" point

 @IBOutlet var fromACT: UITextField!
@IBOutlet var toEXT: UITextField!
@IBOutlet var finalLABEL: UILabel!
func unsafeRandomIntFrom(start: Int32, to end: Int32) -> Int32 {
    let number =  Int32(arc4random_uniform(UInt32(end - start + 1))) + start
    return number
}

@IBAction func random(sender: AnyObject) {

    let convertedNumber = NSString(string: fromACT.text!).intValue
    let convertedNumber2 = NSString(string: toEXT.text!).intValue
    let randomSV = unsafeRandomIntFrom(convertedNumber, to: convertedNumber2)
    let str = String(randomSV)


    finalLABEL.text? = str





}