[FFmpeg-user] converting a video without sound : Error while opening encoder for output stream #0.1

[Access-Dev]

Hello,

I m using FFMPEG to convert some videos that comes from a Wowza 
streaming server.

For some reasons, some videos dont have sound and ffmpeg dont want to 
convert them

Video with sound:

  /usr/bin/ffmpeg -loglevel 0 -i 
/usr/local/WowzaMediaServer-3.0.4/content/167.flv.flv -ar 44100 -b 700k 
-sameq -f avi -r 24 -ab 160k 167.avi


Result :   OK

Stream mapping:
   Stream #0.0 -> #0.0
   Stream #0.1 -> #0.1

Press [q] to stop encoding
frame=  141 fps=  0 q=0.0 Lsize=    1480kB time=10.97 
bitrate=1104.8kbits/s dup=0 drop=8
video:1239kB audio:214kB global headers:0kB muxing overhead 1.800813%

Videos infos:

  Duration: 00:00:11.33, start: 0.000000, bitrate: N/A
     Stream #0.0: Video: flv, yuv420p, 320x200, 1k tbr, 1k tbn, 1k tbc
     Stream #0.1: Audio: nellymoser, 44100 Hz, mono, s16


Video without sound:

/usr/bin/ffmpeg -loglevel 0 -i 
/usr/local/WowzaMediaServer-3.0.4/content/169.flv.flv -ar 44100 -b 700k 
-sameq -f avi -r 24 -ab 160k 169.avi

Stream mapping:
   Stream #0.0 -> #0.0
   Stream #0.1 -> #0.1
Error while opening encoder for output stream #0.1 - maybe incorrect 
parameters such as bit_rate, rate, width or height

Videos infos:

  Duration: 00:00:11.24, start: 0.000000, bitrate: N/A
     Stream #0.0: Video: flv, yuv420p, 320x200, 1k tbr, 1k tbn, 1k tbc
     Stream #0.1: Audio: [0][0][0][0] / 0x0000, 0 channels


How can I tell ffmpeg to create an empty audio track if the source video 
dont have sound ?

Thanks in advance,