Sudden error message in /audio/merge robot

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1

I’ve been using the /audio/merge robot for almost a year now and didn’t have any problems (except initially setting my template up). Since yesterday I keep getting the error message

Invalid audio stream. Exactly one MP3 audio stream is required.

when trying to merge two mp3 files, the relevant part of the code is

    ...
    "merge": {
      "use": {
        "steps": [
          {
            "name": "concatenated",
            "as": "audio"
          },
          {
            "name": "normalized",
            "as": "audio"
          }
        ],
        "bundle_steps": true
      },
      "robot": "/audio/merge",
      "duration": "longest",
      "ffmpeg_stack": "v6.0.0",
      "sign_urls_for": 120
    },
    ...

The only recent change was updating the ffmpeg_stack version from v4.3.1 to the recommended version v6.0.0

2

The problem is with version 6, choosing version 5 fixes it (for now).