[FFmpeg-user] Syntax issue with audio filter

[Marc]

I have a video file that looks like this:

$ ffmpeg -i "input.mkv"
ffmpeg version 2.5.4 Copyright (c) 2000-2015 the FFmpeg developers
  built on Feb 15 2015 21:42:49 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)
---snip---
    Stream #0:0(eng): Video: h264 (High), yuv420p, 1920x800 [SAR 1:1
DAR 12:5], 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
    Stream #0:1(eng): Audio: dts (DTS), 48000 Hz, 5.1(side), fltp,
1536 kb/s (default)
    Stream #0:2(eng): Subtitle: subrip (default)
    Metadata:
      title           : Forced
    Stream #0:3(eng): Subtitle: subrip
    Stream #0:4(eng): Subtitle: subrip
    Metadata:
      title           : SDH

So that's one video stream, one DTS audio stream and three subtitle streams.
I want to create a copy that has all the same streams but add a
two-channel ac3 stream encoded from the existing DTS audio stream.
This is pretty easy to do:

$ ffmpeg -report -loglevel verbose -y -i "input.mkv" -map 0:v:0 -c:v:0
copy -map 0:a:language:eng -c:a:0 copy -map 0:a:language:eng -c:a:1
ac3 -ab 192k -ac 2 -map 0:s -c:s copy "output.mkv"
--snip--
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
  Stream #0:1 -> #0:2 (dts (dca) -> ac3 (native))
  Stream #0:2 -> #0:3 (copy)
  Stream #0:3 -> #0:4 (copy)
  Stream #0:4 -> #0:5 (copy)
Press [q] to stop, [?] for help
q

We can see in the output above that I'm keeping the existing streams
and adding an audio (ac3) stream.

So far so good, but then I wanted to tune the sound in the new stream
with an audio filter, using a single -af parameter:

$ ffmpeg -report -loglevel verbose -y -i "input.mkv" -map 0:v:0 -c:v:0
copy -map 0:a:language:eng -c:a:0 copy -map 0:a:language:eng -c:a:1
ac3 -ab 192k -ac 2 -af
"pan=stereo:FL=FC+0.30*FL+0.30*BL:FR=FC+0.30*FR+0.30*BR" -map 0:s -c:s
copy "output.mkv"
--snip--
Filtergraph 'pan=stereo:FL=FC+0.30*FL+0.30*BL:FR=FC+0.30*FR+0.30*BR'
was defined for audio output stream 0:1 but codec copy was selected.
Filtering and streamcopy cannot be used together.
$ echo $?
1

I don't understand why I'm seeing this error message, it seems that
ffmpeg has confused the two audio streams, the first one is indeed a
copy, but the second (to which I'm applying the filter) is not.

Have I stumbled on a syntax-parsing bug. Or am I just doing something stupid?

Thanks,
Marc.