Closing ticket as duplicate triggers Trac internal error.

[pulkomandy]

Trac detected an internal error:
AttributeError: 'NoneType' object has no attribute 'split'

File "/srv/trac/dev-env/lib/python2.7/site-packages/trac/web/main.py", line 568, in _dispatch_request
File "/srv/trac/dev-env/lib/python2.7/site-packages/trac/web/main.py", line 249, in dispatch
File "/srv/trac/dev-env/lib/python2.7/site-packages/trac/ticket/web_ui.py", line 181, in process_request
File "/srv/trac/dev-env/lib/python2.7/site-packages/trac/ticket/web_ui.py", line 623, in _process_ticket_request
File "/srv/trac/dev-env/lib/python2.7/site-packages/trac/ticket/web_ui.py", line 1355, in _do_save
File "/srv/trac/dev-env/lib/python2.7/site-packages/trac/ticket/model.py", line 369, in save_changes
File "build/bdist.linux-i686/egg/mastertickets/api.py", line 111, in ticket_changed
File "build/bdist.linux-i686/egg/mastertickets/model.py", line 67, in save

The changes are saved but I land on an error page.