git pull --ff-only fail

[zqfan]

Traceback (most recent call last):
File "/usr/local/bin/gitall", line 11, in
sys.exit(main())
File "/usr/local/lib/python2.7/dist-packages/gitall/gitall.py", line 34, in main
git.cmd.Git(dest_dir).pull(ff_only=True)
File "/usr/local/lib/python2.7/dist-packages/git/cmd.py", line 459, in
return lambda _args, *_kwargs: self._call_process(name, _args, *_kwargs)
File "/usr/local/lib/python2.7/dist-packages/git/cmd.py", line 920, in _call_process
return self.execute(make_call(), **_kwargs)
File "/usr/local/lib/python2.7/dist-packages/git/cmd.py", line 705, in execute
raise GitCommandError(command, status, stderr_value)
git.exc.GitCommandError: 'git pull --ff-only' returned with exit code 1
stderr: 'From https://github.com/openstack/charm-nova-cloud-controller
854e3e7..30c6031 master -> origin/master
error: 'refs/remotes/origin/stable' exists; cannot create 'refs/remotes/origin/stable/16.04'
! [new branch] stable/16.04 -> origin/stable/16.04 (unable to update local ref)
error: some local refs could not be updated; try running
'git remote prune origin' to remove any old, conflicting branches'