c2s2127010 error Traceback[c2s]

I've the following log entries, but I don't know why and I can't repruduce it.
I'm running Prosody trunk nightly build 552 (2015-03-27, d2faaaca695d) on 
centos 6.6

In the error log there are the following lines, they repeat round about once a 
week:

Apr 17 19:23:47 c2s2127010  error   Traceback[c2s]: 
.../.toast/armed/lib/prosody/modules/muc/muc.lib.lua:726: attempt to index 
local 'item' (a nil value)
stack traceback:
    .../.toast/armed/lib/prosody/modules/muc/muc.lib.lua:726: in function <.../.toast/armed/lib/prosody/modules/muc/muc.lib.lua:724>
    (tail call): ?
    .../.toast/armed/lib/prosody/util/events.lua:67: in function <.../.toast/armed/lib/prosody/util/events.lua:63>
    (tail call): ?
    (tail call): ?
    .../.toast/armed/lib/prosody/util/events.lua:67: in function 'fire_event'
    .../.toast/armed/lib/prosody/core/stanza_router.lua:187: in function 'core_post_stanza'
    .../.toast/armed/lib/prosody/core/stanza_router.lua:135: in function 'core_process_stanza'
    .../.toast/armed/lib/prosody/modules/mod_c2s.lua:236: in function 'func'
    .../.toast/armed/lib/prosody/util/async.lua:88: in function <.../.toast/armed/lib/prosody/util/async.lua:86>

Any ideas?

Original issue reported on code.google.com by christia...@pix-art.de on 17 Apr 2015 at 6:11

baiwyc119 / lxmppd

c2s2127010 error Traceback[c2s] #491