ReadOnlyError when using tag for bzr viz

I get a ReadOnlyError when I use the tag function form bzr viz, but *not* when in the log window in olive-gtk.

Steps to reproduce:
* Run bzr viz
* Right click on a revision, click on tag revision.
* Enter tag name.
* Click Add Tag

Expected result:
* Tag added.

Actual Result:
* The following error is written to the console, and the tag is not added:
Traceback (most recent call last):
  File bzrlib/plugins/gtk/revisionmenu.py, line 84, in show_tag
    self.branch.lock_write()
  File bzrlib/branch.py, line 1304, in lock_write
    repo_token = self.repository.lock_write()
  File bzrlib/repository.py, line 306, in lock_write
    result = self.control_files.lock_write(token=token)
  File bzrlib/lockable_files.py, line 225, in lock_write
    raise errors.ReadOnlyError(self)
bzrlib.errors.ReadOnlyError: A write attempt was made in a read only transaction on LockableFiles(lock, /path/to/.bzr/repository/)