Impossible return value from oggz_stream_get_content()

This is the documentation of oggz_stream_get_content():

/**
 * Determine the content type of the oggz stream referred to by \a serialno
 *
 * \param oggz An OGGZ handle
 * \param serialno An ogg stream serialno
 * \retval OGGZ_CONTENT_THEORA..OGGZ_CONTENT_UNKNOWN content successfully 
 *          identified
 * \retval OGGZ_ERR_BAD_OGGZ \a oggz does not refer to an existing OGGZ
 * \retval OGGZ_ERR_BAD_SERIALNO \a serialno does not refer to an existing
 *          stream
 */
OggzStreamContent oggz_stream_get_content (OGGZ * oggz, long serialno);

The problem is that OggzStreamContent is an enum that do not contain OGGZ_ERR_BAD_OGGZ nor OGGZ_ERR_BAD_SERIALNO, so those return values are "impossible" and cause a compiler warning:

oggz_stream.c: In function ‘oggz_stream_get_content’:
oggz_stream.c:55:28: warning: implicit conversion from ‘enum OggzError’ to ‘OggzStreamContent’ [-Wenum-conversion]
   55 |   if (oggz == NULL) return OGGZ_ERR_BAD_OGGZ;
      |                            ^~~~~~~~~~~~~~~~~
oggz_stream.c:58:30: warning: implicit conversion from ‘enum OggzError’ to ‘OggzStreamContent’ [-Wenum-conversion]
   58 |   if (stream == NULL) return OGGZ_ERR_BAD_SERIALNO;
      |                              ^~~~~~~~~~~~~~~~~~~~~