bzr_dir_for_branch_stacked_on_relative_url.clone('...', preserve_stacking=True) miscalculates stacked-on URL

I hope this transcript makes the summary sufficiently precise:

$ cd ~/tmp
$ mkdir repro
$ cd repro/
$ mkdir directory
$ bzr init directory/base --development
$ bzr push -d directory/base/ --stacked-on=directory/base directory/branch
Created new stacked branch referring to file:///home/mwh/tmp/repro/directory/base.
$ cat directory/branch/.bzr/branch/branch.conf
stacked_on_location = file:///home/mwh/tmp/repro/directory/base
$ echo 'stacked_on_location = ../base' > directory/branch/.bzr/branch/branch.conf
$ bzr info directory/branch/
Standalone branch (format: development1)
Location:
  branch root: directory/branch

Related branches:
  stacked on: ../base
$ python2.4
Python 2.4.5 (#2, Apr 21 2008, 11:22:42)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import bzrlib.branch
>>> b = bzrlib.branch.Branch.open('directory/branch')
>>> b.bzrdir.clone('dest', preserve_stacking=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/usr/lib/python2.4/site-packages/bzrlib/bzrdir.py", line 177, in clone
    preserve_stacking=preserve_stacking)
  File "/usr/lib/python2.4/site-packages/bzrlib/bzrdir.py", line 222, in clone_on_transport
    result_repo = repository_policy.acquire_repository(
  File "/usr/lib/python2.4/site-packages/bzrlib/bzrdir.py", line 2863, in acquire_repository
    self._add_fallback(repository)
  File "/usr/lib/python2.4/site-packages/bzrlib/bzrdir.py", line 2817, in _add_fallback
    stacked_dir = BzrDir.open(stack_on)
  File "/usr/lib/python2.4/site-packages/bzrlib/bzrdir.py", line 763, in open
    return BzrDir.open_from_transport(t, _unsupported=_unsupported)
  File "/usr/lib/python2.4/site-packages/bzrlib/bzrdir.py", line 800, in open_from_transport
    redirected)
  File "/usr/lib/python2.4/site-packages/bzrlib/transport/__init__.py", line 1616, in do_catching_redirections
    return action(transport)
  File "/usr/lib/python2.4/site-packages/bzrlib/bzrdir.py", line 776, in find_format
    return transport, BzrDirFormat.find_format(
  File "/usr/lib/python2.4/site-packages/bzrlib/bzrdir.py", line 1573, in find_format
    raise errors.NotBranchError(path=transport.base)
bzrlib.errors.NotBranchError: Not a branch: "/home/mwh/tmp/base/".
>>>

This is with 1.6b3 but it's the same with bzr.dev revno 3565.