Post Request with AFHTTPSessionManager Stripe - Swift -

i trying make post request server in order process card transaction through stripe, when press paybuttonaction error "terminating app due uncaught exception 'nsinvalidargumentexception', reason: '*** -[__nscfconstantstring stringbyappendingstring:]: nil argument'" , redirected simulator -ios 10.1>usr/include>dispatch/once.h>no selection dispatch_once(predicate, block); thread 1: signal sigbart . tested button without code , works. can tell me doing wrong

 //create stripe payment field let paymenttextfield = stppaymentcardtextfield() @iboutlet weak var paybuttonoutlet: uibutton! @ibaction func paybuttonaction(sender: uibutton) {      let card = paymenttextfield.cardparams     stpapiclient.sharedclient().createtokenwithcard(card) { (token, error) in         if let error = error {             print(error)         } else if let token = token {             print("the token \(token)")             self.chargeusingtoken(token) // post our server         }     } }  // post parameters our webserver make further requests server func chargeusingtoken(token:stptoken) {     let urlstring = "0.0.0.0:8080/charge/stripe"     let params = ["stripetoken":token.tokenid, "amount":fulldata.finalbookingamount, "currency":"gbp", "description":fulldata.finalemailaddress]      //send post request server     let manager = afhttpsessionmanager()      manager.post(urlstring, parameters: params, progress: { (progress) in         //         }, success: { (requestoperation, response) in             if let response = response as? [string: string] {                 print(response)             }          }) { (requestoperation, error) in                 print(error)     }  }