panic failed to add item

I get this running mnogosearch against my 8.4.1 database consistently:

PANIC:  failed to add item to the right sibling in index "logged_in_uid"
STATEMENT:  INSERT INTO logged_in (orgid, uid, remote_addr,
orig_session_id, new_session_id) VALUES ('394746', '1125200',
'24.251.180.193', 'c2f5bfe61be2ea80548c46c156f2242d',
'2e777119d2bf0d5e35b653b80f3804e8');
LOG:  server process (PID 27524) was terminated by signal 6: Aborted
LOG:  terminating any other active server processes

When I run against an 8.3.8 database I do not.  It takes days to see
this error and it scrams whatever indexer is running at the time.  for
now I don't have time to do a lot of testing, but if anyone wants me
to try anything to track it down or something I'm all ears.  And
possibly left thumbs.