NSData base64Encode return nil swift

i tryed

let contents = results["contents"] as! String!
let decodedData = NSData(base64EncodedString: contents, 
options:NSDataBase64DecodingOptions())
let decodedString = NSString(data: decodedData!, encoding: NSUTF8StringEncoding)





let contents = results["contents"] as! String!
let decodedData = NSData(base64EncodedString: contents, options:NSDataBase64DecodingOptions.IgnoreUnknownCharacters)
let decodedString = NSString(data: decodedData!, encoding: NSUTF8StringEncoding)



let contents = results["contents"] as! String!
let decodedData = NSData(base64EncodedString: contents, options:NSDataBase64DecodingOptions(rawValue: 0)!)
let decodedString = NSString(data: decodedData!, encoding: NSUTF8StringEncoding)

always EXC_BAC_INSTRUCTION from decodedString maybe decodedData retu nil

why NSData retu nil