输出在AVExportSession中显示为null

Question

我无法获得视频文件的输出，

    self.exportSession = [[AVAssetExportSession alloc]
                          initWithAsset:self.videoAsset presetName:AVAssetExportPresetPassthrough];
    // Implementation continues.



     self.exportSession.outputURL = furl;

self.videoAsset是：

<AVURLAsset: 0x7faf62e65000, URL = https://video.xx.fbcdn.net/v/t43.1792-2/12529221_1698805023741577_254337559_n.mp4?efg=eyJybHIiOjE1MDAsInJsYSI6MjM3MywidmVuY29kZV90YWciOiJzdmVfaGQifQ%3D%3D&rl=1500&vabr=365&oh=4c6901578cd1316b5d2c38a127d8f165&oe=571B3BC3>