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Using either an uncoupled stereo signal or any other singal type that lends itself to utilizing the multistream APIs, when setting the bitrate to Auto (-1000) or Max (-1) by the multistream_ctl function or by setting a very low bitrate, relative to the number of channels. This error only happens with libopus v1.1 but not any version before, including libopus v1.1-beta. This can be replicated with the opus tools 0.1.8-win32 encoder with the following example commands:

C:\opus-tools-0.1.8-win32>opusenc --set-ctl-int 4002=-1000 --hard-cbr Quadraphonic.wav Quadraphonic.opus
Encoding using libopus 1.1 (audio)
-----------------------------------------------------
   Input: 48kHz 4 channels
  Output: 4 channels (4 coupled)
          20ms packets, 192kbit/sec CBR
 Preskip: 312

Encoding failed: invalid argument. Aborting.
Encoding complete
-----------------------------------------------------
       Encoded:
       Runtime: 1e-006 seconds
                (0x realtime)
         Wrote: 847 bytes, 1 packets, 2 pages
       Bitrate: -1.#INDkbit/s (without overhead)
 Instant rates: 3065.6kbit/s to 0kbit/s
                (7664 to 0 bytes per packet)
      Overhead: 100% (container+metadata)


C:\opus-tools-0.1.8-win32>opusenc --set-ctl-int 4002=-1 --hard-cbr Quadraphonic.wav Quadraphonic.opus
Encoding using libopus 1.1 (audio)
-----------------------------------------------------
   Input: 48kHz 4 channels
  Output: 4 channels (4 coupled)
          20ms packets, 192kbit/sec CBR
 Preskip: 312

Encoding failed: invalid argument. Aborting.
Encoding complete
-----------------------------------------------------
       Encoded:
       Runtime: 1e-006 seconds
                (0x realtime)
         Wrote: 847 bytes, 1 packets, 2 pages
       Bitrate: -1.#INDkbit/s (without overhead)
 Instant rates: 3065.6kbit/s to 0kbit/s
                (7664 to 0 bytes per packet)
      Overhead: 100% (container+metadata)


C:\opus-tools-0.1.8-win32>opusenc --set-ctl-int 4002=1599 --hard-cbr Quadraphonic.wav Quadraphonic.opus
Encoding using libopus 1.1 (audio)
-----------------------------------------------------
   Input: 48kHz 4 channels
  Output: 4 channels (4 coupled)
          20ms packets, 192kbit/sec CBR
 Preskip: 312

Encoding failed: invalid argument. Aborting.
Encoding complete
-----------------------------------------------------
       Encoded:
       Runtime: 1e-006 seconds
                (0x realtime)
         Wrote: 847 bytes, 1 packets, 2 pages
       Bitrate: -1.#INDkbit/s (without overhead)
 Instant rates: 3065.6kbit/s to 0kbit/s
                (7664 to 0 bytes per packet)
      Overhead: 100% (container+metadata)


C:\opus-tools-0.1.8-win32>

The above commands do not show encoding errors with previous versions of the tools.

Attached quadraphonic sample audio file for testing.