ffmpeg error Output file #0 does not contain any stream

[lijiansong]

when I run the following command:
ffmpeg -i fox.mp4 fox.mp3
in Ubuntu, ffmpeg dump error msg,

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'fox.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf55.48.100
  Duration: 00:00:14.02, start: 0.000000, bitrate: 887 kb/s
    Stream #0:0(und): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuv420p, 640x640 [SAR 1:1 DAR 1:1], 886 kb/s, 29.97 fps, 29.97 tbr, 11988 tbn, 59.94 tbc (default)
    Metadata:
      handler_name    : VideoHandler
Output #0, mp3, to 'fox.mp3':
Output file #0 does not contain any stream