get_branch_reference fails in a new BzrDirMeta1Colo

I would like to use get_branch_reference in a newly created colocated workspace to get the current branch name, but it fails with an error:

$ bzr init --development-colo file:test,branch=trunk
Created a lightweight checkout (format: development-colo)
$ cd test
$ PYTHONPATH=$HOME/src/bzr/bzr.dev python -c "from bzrlib import bzrdir; d=bzrdir.BzrDir.open('.'); d.get_branch_reference(name=None)"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/nmb/src/bzr/bzr.dev/bzrlib/bzrdir.py", line 855, in get_branch_reference
    format = BranchFormat.find_format(self, name=name)
  File "/Users/nmb/src/bzr/bzr.dev/bzrlib/branch.py", line 1615, in find_format
    raise errors.NotBranchError(path=transport.base, bzrdir=controldir)
bzrlib.errors.NotBranchError: Not a branch: "/Users/nmb/tmp/test/.bzr/branch/": location is a repository.