`bzr qdi --encoding xxx` is broken

bzr 2.2.1, QBzr 0.19 revno 1322.

`bzr qdiff --encoding xxx` fails with following traceback (from .bzr.log):

0.188 bazaar version: 2.2.1
0.188 bzr arguments: [u'qdi', u'--encoding', u'cp1251']
0.203 looking for plugins in C:\work\Bazaar\plugins
0.360 looking for plugins in C:/Program Files/Bazaar/plugins
0.594 encoding stdout as sys.stdout encoding 'cp866'
1.000 opening working tree 'C:/work/MyCode/logic'
1.031 Traceback (most recent call last):
  File "C:\work\Bazaar\plugins\qbzr\lib\trace.py", line 300, in reports_exception_decorate
  File "C:\work\Bazaar\plugins\qbzr\lib\diffwindow.py", line 235, in initial_load
  File "bzrlib\cleanup.pyo", line 131, in run
  File "bzrlib\cleanup.pyo", line 165, in _do_with_cleanups
  File "C:\work\Bazaar\plugins\qbzr\lib\diffwindow.py", line 247, in _initial_load
  File "C:\work\Bazaar\plugins\qbzr\lib\diffwindow.py", line 253, in load_branch_info
  File "C:\work\Bazaar\plugins\qbzr\lib\util.py", line 611, in get_set_encoding
  File "bzrlib\config.pyo", line 754, in set_user_option
  File "bzrlib\config.pyo", line 952, in set_option
  File "bzrlib\branch.pyo", line 2452, in lock_write
  File "bzrlib\lockable_files.pyo", line 182, in lock_write
ReadOnlyError: A write attempt was made in a read only transaction on LockableFiles(lock, file:///C:/work/MyCode/logic/.bzr/branches/refactoring/.bzr/branch/)

Changing encoding via GUI works as expected.