Trac failure: "KeyError: 'fields'"

How to Reproduce

While doing a GET operation on /ticket/4000, Trac issued an internal error.

Request parameters:

{'action': u'diff', 'id': u'4000', 'version': u'3'}

User Agent was: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:5.0) Gecko/20100101 Firefox/5.0

System Information

**Trac**	`0.11.6`
**Python**	`2.5.2 (r252:60911, Jan 24 2010, 18:02:01) ` \[\[br\]\] `[GCC 4.3.2]`
**setuptools**	`0.6c8`
**SQLite**	`3.5.9`
**pysqlite**	`2.3.2`
**Genshi**	`0.5.1`
**mod_python**	`3.3.1`
**Pygments**	`0.10`
**GIT**	`1.5.6.5`
**FullBlog**	`0.1.1`
**RPC**	`1.0.6`
**CustomFieldAdmin**	`0.2.2`
**jQuery:**	`1.2.6`

Python Traceback

Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/trac/web/main.py", line 450, in _dispatch_request
    dispatcher.dispatch(req)
  File "/usr/lib/python2.5/site-packages/trac/web/main.py", line 217, in dispatch
    self._post_process_request(req, *resp)
  File "/usr/lib/python2.5/site-packages/trac/web/main.py", line 309, in _post_process_request
    resp = f.post_process_request(req, *resp)
  File "build/bdist.linux-x86_64/egg/mastertickets/web_ui.py", line 64, in post_process_request
    for field, field_data in change['fields'].iteritems():
KeyError: 'fields'

Trac metadata

Trac field	Value
Version	7.0.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	None
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