mkdocs: Error while executing gh-deploy

I’ve successfully deployed a MkDocs site using the gh-deploy command. When I try to deploy some additional changes to my master branch, I get the following error:

c:\docs>mkdocs gh-deploy --clean
INFO    -  Cleaning site directory
INFO    -  Building documentation to directory: c:\docs\site
INFO    -  Copying 'c:\docs\site' to 'gh-pages' branch and pushing to GitHub.
Traceback (most recent call last):
  File "C:\Python34\lib\runpy.py", line 170, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\Python34\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "c:\Python34\Scripts\mkdocs.exe\__main__.py", line 9, in <module>
  File "C:\Python34\lib\site-packages\click\core.py", line 664, in __call__
    return self.main(*args, **kwargs)
  File "C:\Python34\lib\site-packages\click\core.py", line 644, in main
    rv = self.invoke(ctx)
  File "C:\Python34\lib\site-packages\click\core.py", line 991, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "C:\Python34\lib\site-packages\click\core.py", line 837, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "C:\Python34\lib\site-packages\click\core.py", line 464, in invoke
    return callback(*args, **kwargs)
  File "C:\Python34\lib\site-packages\mkdocs\cli.py", line 186, in gh_deploy_command
    gh_deploy.gh_deploy(config, message=message)
  File "C:\Python34\lib\site-packages\mkdocs\gh_deploy.py", line 69, in gh_deploy
    remote_branch)
  File "C:\Python34\lib\site-packages\mkdocs\utils\ghp_import.py", line 163, in ghp_import
    if not try_rebase(remote, branch):
  File "C:\Python34\lib\site-packages\mkdocs\utils\ghp_import.py", line 78, in try_rebase
    if sp.call(cmd) != 0:
  File "C:\Python34\lib\subprocess.py", line 537, in call
    with Popen(*popenargs, **kwargs) as p:
  File "C:\Python34\lib\subprocess.py", line 859, in __init__
    restore_signals, start_new_session)
  File "C:\Python34\lib\subprocess.py", line 1086, in _execute_child
    args = list2cmdline(args)
  File "C:\Python34\lib\subprocess.py", line 663, in list2cmdline
    needquote = (" " in arg) or ("\t" in arg) or not arg
TypeError: 'str' does not support the buffer interface