Error when adding silent channels to ProRes file in FFMpeg

Question

I'm trying to follow this guide to build a usable 5.1 audio for a piece of hardware with specific needs.

When I run the command:

ffmpeg -ar 48000 -acodec pcm_s24le -f s24le -ac 5 -i /dev/zero -i file_with_5.1_audio.mov -vcodec copy -acodec pcm_s24le -map 1:1 -filter_complex "[0:0][0:0]amerge=inputs=2[aout];[1:0][aout]amerge=inputs=2[afin]" -map [afin] -ac 16 output.mov

as shown, I'm getting the following error:

Stream specifier ':0' in filtergraph description [0:0][0:0]amerge=inputs=2[aout];[1:0][aout]amerge=inputs=2[afin] matches no streams.

Here's my output when attempting to run FFMpeg prior to error:

    ffmpeg version N-73935-g5362df2 Copyright (c) 2000-2015 the FFmpeg developers
built with Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
configuration: --prefix=/Volumes/tempdisk/sw --enable-gpl --enable-pthreads --disable-ffplay --disable-ffserver --enable-libvpx --disable-decoder=libvpx --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libx264 --enable-libxvid --enable-avfilter --enable-zlib --enable-filters --arch=x86 --enable-runtime-cpudetect
libavutil      54. 28.100 / 54. 28.100
libavcodec     56. 51.100 / 56. 51.100
libavformat    56. 40.101 / 56. 40.101
libavdevice    56.  4.100 / 56.  4.100
libavfilter     5. 27.100 /  5. 27.100
libswscale      3.  1.101 /  3.  1.101
libswresample   1.  2.101 /  1.  2.101
libpostproc    53.  3.100 / 53.  3.100
Guessed Channel Layout for  Input Stream #0.0 : 5.0
Input #0, s24le, from '/dev/zero':
Duration: N/A, bitrate: 5760 kb/s
Stream #0:0: Audio: pcm_s24le, 48000 Hz, 5 channels, s32 (24 bit), 5760 kb/s
Input #1, mov,mp4,m4a,3gp,3g2,mj2, from '/Volumes/PathToFile.mov':
Metadata:
major_brand     : qt  
minor_version   : 537199360
compatible_brands: qt  
creation_time   : 2015-07-22 21:22:49
Duration: 00:00:37.54, start: 0.000000, bitrate: 26106 kb/s
Stream #1:0(eng): Video: prores (apch / 0x68637061), yuv422p10le(bt709), 1920x1080, 18971 kb/s, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 24k tbn, 24k tbc (default)
Metadata:
  creation_time   : 2015-07-22 21:22:49
  handler_name    : Apple Alias Data Handler
  encoder         : Apple ProRes 422 (HQ)
  timecode        : 00:00:00:00
Stream #1:1(eng): Audio: pcm_s24le (lpcm / 0x6D63706C), 48000 Hz, 5.1, s32 (24 bit), 6912 kb/s (default)
Metadata:
  creation_time   : 2015-07-22 21:22:49
  handler_name    : Apple Alias Data Handler
  timecode        : 00:00:00:00
Stream #1:2(eng): Data: none (tmcd / 0x64636D74) (default)
Metadata:
  creation_time   : 2015-07-22 21:22:49
  handler_name    : Apple Alias Data Handler
  timecode        : 00:00:00:00

Any help would be greatly appreciated, I've been working on this off and on for weeks, and feel so close.