title: FFmpeg 'Unable to parse option value "(null)" as sample format'
body: <p>Im trying to process a file and run the following command</p>
  
  <pre><code>ffmpeg -i input.webm output.webm
  </code></pre>
  
  <p>I'm doing it with the ffmpeg library from <a href="https://bgrins.github.io/videoconverter.js/" rel="nofollow">videoconverter.js</a>. I'm trying to understand what is wrong or how I can fix it.</p>
  
  <p>I end up getting this:</p>
  
  <pre><code>Worker has received command
  Received command: -i input.webm output.webm.  Processing with 268435456 bits.
  ffmpeg version 2.2.1 Copyright (c) 2000-2014 the FFmpeg developers
    built on Jun  9 2014 20:24:32 with emcc (Emscripten GCC-like replacement) 1.12.0 (commit 6960d2296299e96d43e694806f5d35799ef8d39c)
    configuration: --cc=emcc --prefix=/Users/bgrinstead/Sites/videoconverter.js/build/ffmpeg/../dist --extra-cflags='-I/Users/bgrinstead/Sites/videoconverter.js/build/ffmpeg/../dist/include -v' --enable-cross-compile --target-os=none --arch=x86_32 --cpu=generic --disable-ffplay --disable-ffprobe --disable-ffserver --disable-asm --disable-doc --disable-devices --disable-pthreads --disable-w32threads --disable-network --disable-hwaccels --disable-parsers --disable-bsfs --disable-debug --disable-protocols --disable-indevs --disable-outdevs --enable-protocol=file --enable-libvpx --enable-gpl --extra-libs='/Users/bgrinstead/Sites/videoconverter.js/build/ffmpeg/../dist/lib/libx264.a /Users/bgrinstead/Sites/videoconverter.js/build/ffmpeg/../dist/lib/libvpx.a'
    libavutil      52. 66.100 / 52. 66.100
    libavcodec     55. 52.102 / 55. 52.102
    libavformat    55. 33.100 / 55. 33.100
    libavdevice    55. 10.100 / 55. 10.100
    libavfilter     4.  2.100 /  4.  2.100
    libswscale      2.  5.102 /  2.  5.102
    libswresample   0. 18.100 /  0. 18.100
    libpostproc    52.  3.100 / 52.  3.100
  [vp8 @ 0xed8c00] Warning: not compiled with thread support, using thread emulation
  Guessed Channel Layout for  Input Stream #0.0 : mono
  Input #0, matroska,webm, from 'input.webm':
    Metadata:
      encoder         : Chrome
    Duration: N/A, start: 0.000000, bitrate: N/A
      Stream #0:0(eng): Audio: opus, 48000 Hz, mono (default)
      Stream #0:1(eng): Video: vp8, yuv420p, 640x480, SAR 1:1 DAR 4:3, 30 fps, 30 tbr, 1k tbn, 1k tbc (default)
  [abuffer @ 0xedd670] Unable to parse option value "(null)" as sample format
      Last message repeated 1 times
      Last message repeated 1 times
  [abuffer @ 0xedd670] Error setting option sample_fmt to value (null).
  [graph 1 input from stream 0:0 @ 0xedd600] Error applying options to the filter.
  Error opening filters!
  Finished processing (took 673ms)
  </code></pre>
  
  <p>The end result is stopped due to 'Unable to parse option value "(null)" as sample format'. How would I solve this?</p>