Randomly internal error in 'Timeline' menu, 'argument to += must be iterable'

[daybreaker12@…]

I've installed trac 0.9.4 in CentOS(kernel 2.6.9), with Python 2.3. The following error is occurred occasionally(not always), when just clicking 'Timeline' menu.

Error url : http://lint.tattersite.com/timeline

---

Oops...
Trac detected an internal error:

argument to += must be iterable

Python traceback

Traceback (most recent call last):
  File "/usr/lib/python2.3/site-packages/trac/web/modpython_frontend.py", line 206, in handler
    dispatch_request(mpr.path_info, mpr, env)
  File "/usr/lib/python2.3/site-packages/trac/web/main.py", line 139, in dispatch_request
    dispatcher.dispatch(req)
  File "/usr/lib/python2.3/site-packages/trac/web/main.py", line 107, in dispatch
    resp = chosen_handler.process_request(req)
  File "/usr/lib/python2.3/site-packages/trac/Timeline.py", line 140, in process_request
    filters)
TypeError: argument to += must be iterable

[eblot]

Duplicate of #2965.