Erro de formato de amostra inválido ao converter um arquivo iso de DVD para Mp4

Eu recebo o seguinte erro ao converter um arquivo ISO para MP4. O arquivo ISO contém um diretório VIDEO_TS com os arquivos de vídeo:

ffmpeg -i DieRelativeringVanMenswees.iso DieRelativeringVanMenswees.mp4

ffmpeg version git-2013-04-23-b21b2ca Copyright (c) 2000-2013 the FFmpeg developers
  built on Apr 23 2013 20:19:05 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --enable-gpl --enable-libass --enable-libfaac --enable-libfdk-aac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libspeex --enable-librtmp --enable-libtheora --enable-libvorbis --enable-libvpx --enable-x11grab --enable-libx264 --enable-nonfree --enable-version3
  libavutil      52. 27.100 / 52. 27.100
  libavcodec     55.  6.100 / 55.  6.100
  libavformat    55.  3.100 / 55.  3.100
  libavdevice    55.  0.100 / 55.  0.100
  libavfilter     3. 60.101 /  3. 60.101
  libswscale      2.  2.100 /  2.  2.100
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  3.100 / 52.  3.100
[mpeg @ 0xa48cc60] max_analyze_duration 5000000 reached at 5000000 microseconds
Guessed Channel Layout for  Input Stream #0.4 : stereo
Input #0, mpeg, from 'DieRelativeringVanMenswees.iso':
  Duration: 00:51:09.09, start: 0.261178, bitrate: 10776 kb/s
    Stream #0:0[0x1bf]: Data: dvd_nav_packet
    Stream #0:1[0x1e0]: Video: mpeg2video (Main), yuv420p, 720x576 [SAR 64:45 DAR 16:9], 25 fps, 25 tbr, 90k tbn, 50 tbc
    Stream #0:2[0x20]: Subtitle: dvd_subtitle
    Stream #0:3[0x21]: Subtitle: dvd_subtitle
    Stream #0:4[0xa0]: Audio: pcm_s16be, 48000 Hz, stereo, 1536 kb/s
[graph 1 input from stream 0:4 @ 0xa49a420] Invalid sample format (null)
Error opening filters!