What version are you running?
2.6 alpha 0 (dev)
What steps will reproduce the problem?

Run Unit Tests

What is the expected output? What do you see instead?
Expected: Pass
Output:
ERROR: Testing a SFTP-backed bzr repository
Traceback (most recent call last):

  File "/src/reviewboard/reviewboard/scmtools/tests.py", line 444, in test_sftp

    self._test_ssh(self.bzr_sftp_path, 'README')

  File "/src/reviewboard/reviewboard/scmtools/tests.py", line 79, in _test_ssh

    tool.check_repository(repo_path)

  File "/src/reviewboard/reviewboard/scmtools/bzr.py", line 247, in check_repository

    raise SCMError(e)

SCMError: prefetch() takes exactly 2 arguments (1 given)

Please provide any additional information below.
Using Vagrant box provided by Steven Macleod.