Audio KeyError

[ephes]

Probably a minor issue - here's the traceback:

KeyError: 'audio'
  File "django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
  File "wagtail/models/__init__.py", line 678, in _get_response
    response = obj.serve_preview(request, preview_mode)
  File "wagtail/models/__init__.py", line 811, in serve_preview
    self.get_preview_context(request, mode_name),
  File "wagtail/models/__init__.py", line 1925, in get_preview_context
    return self.get_context(request)
  File "cast/models/pages.py", line 333, in get_context
    context["audio_items"] = self.media_lookup["audio"].items()