return NSDate will in swift 2 : when 
When I change my streng to Nsdate , return nill 

and show error : 

> fatal error: unexpectedly found nil while unwrapping an Optional value

 for example :     date = 8/9/2016 9:45:19 AM


    convertDate(DateString:String) -> NSDate{
            let dateFormatter = NSDateFormatter()
            dateFormatter.dateFormat = "MM/dd/yyyy hh:mm:ss a"
            let date = dateFormatter.dateFromString(DateString)
            print(date) // return nil
    
            return date! 
        }

You can guide what is the reason?