I can't trim the video with type of mp4. How to resolve it?

Hello,
      Thank you for providing us so good open source code. I met with a problem and help you could help me.
      My video type is mp4, and when run to code " Float64 duration = CMTimeGetSeconds([self.asset duration]);". It said that asset can't count mp4's duration. So how can I do? Must I transfer Mp4 to .mov?
   Thanks a lot! 