FTS indexer-worker Panic

Set up fts_xapian over the weekend and re-indexed. https://github.com/grosjo/fts-xapian

Tried to search my INBOX and got:

dovecot: indexer-worker: Panic: file charset-iconv.c: line 83 (charset_to_utf8_try): assertion failed: (srcleft <= CHARSET_MAX_PENDING_BUF_SIZE)

What could I possibly have lurking in my INBOX to cause that ??

-- Yarema