doveadm index assertion failed

[Nick Rosier]

Hi,

one of my mailboxes returns following error when I run doveadm index on 
it:

Panic: file charset-iconv.c: line 85 (charset_to_utf8_try): assertion 
failed: (srcleft <= CHARSET_MAX_PENDING_BUF_SIZE)

OS: FreeBSD 10.2
Dovecot: 2.1.19
Tika: 1.10
SOLR: 5.3.1

Running doveadm -D index does not show any more information indicating 
what causes this error (mail/folder/...)
How I can find what is causing this error?

Rgds,
N.