ios - Can't stop audioRecorder -

here's part of code:

class audiorecordercontroller: uiviewcontroller, avaudiorecorderdelegate {

var audiorecorder: avaudiorecorder?  func audiorecordingpath() -> nsurl {      let filemanager = nsfilemanager()      let documentsfolderurl = filemanager.urlfordirectory(.documentdirectory, indomain: .userdomainmask, appropriateforurl: nil, create: false, error: nil)      return documentsfolderurl!.urlbyappendingpathcomponent("recording.m4a")  }   func audiorecordingsettings() -> nsdictionary {...}  func startrecordingaudio() {      var error: nserror?      let audiorecordingurl = self.audiorecordingpath()      audiorecorder = avaudiorecorder(url: audiorecordingurl, settings: audiorecordingsettings() [nsobject: anyobject], error: &error)      if let recorder = audiorecorder {          recorder.delegate = self          if recorder.preparetorecord() && recorder.record() {              println("capture succeed!")              let delayinseconds = 5.0             let delayinnanoseconds = dispatch_time(dispatch_time_now, int64(delayinseconds * double(nsec_per_sec)))             dispatch_after(delayinnanoseconds, dispatch_get_main_queue(), {                 [weak self] in                 self!.audiorecorder!.stop()                 //it breaks here.                  })          }      } } 

it printed "capture succeed" in console, crashed error "unexpectedly found nil while unwrapping optional value". what's problem last line of code?

have tried replacing

self!.audiorecorder!.stop()

with using unwrapped constant

recorder.stop()