tmuxp TMUX= behaviour inconsistent with tmux

Launching/attaching tmux via TMUX= tmuxp load ... prompts the user about attaching if the target session already exists, is already running. Attach? [Y/n]:. When attachment is confirmed, it complains with the below traceback, whereas env -u TMUX tmuxp load ... is successful.

For tmux, TMUX= tmux attach attaches successfully.

Traceback:

Traceback (most recent call last):
  File "/nix/store/ck0c5r9lzb05ly2z4d9gbjkd7gdyqgpz-tmuxp-1.5.4/bin/.tmuxp-wrapped", line 9, in <module>
    sys.exit(cli.cli())
  File "/nix/store/b1pz8zmx76gay0aj939n4irnbqwc77sf-python2.7-click-7.0/lib/python2.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/nix/store/b1pz8zmx76gay0aj939n4irnbqwc77sf-python2.7-click-7.0/lib/python2.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/nix/store/b1pz8zmx76gay0aj939n4irnbqwc77sf-python2.7-click-7.0/lib/python2.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/nix/store/b1pz8zmx76gay0aj939n4irnbqwc77sf-python2.7-click-7.0/lib/python2.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/nix/store/b1pz8zmx76gay0aj939n4irnbqwc77sf-python2.7-click-7.0/lib/python2.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/nix/store/b1pz8zmx76gay0aj939n4irnbqwc77sf-python2.7-click-7.0/lib/python2.7/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/nix/store/ck0c5r9lzb05ly2z4d9gbjkd7gdyqgpz-tmuxp-1.5.4/lib/python2.7/site-packages/tmuxp/cli.py", line 814, in command_load
    load_workspace(config[-1], **tmux_options)
  File "/nix/store/ck0c5r9lzb05ly2z4d9gbjkd7gdyqgpz-tmuxp-1.5.4/lib/python2.7/site-packages/tmuxp/cli.py", line 527, in load_workspace
    _reattach(builder.session)
  File "/nix/store/ck0c5r9lzb05ly2z4d9gbjkd7gdyqgpz-tmuxp-1.5.4/lib/python2.7/site-packages/tmuxp/cli.py", line 390, in _reattach
    session.switch_client()
  File "/nix/store/2mqsnqhzrs4v2qqp8bzbbm3frsdq8bch-python2.7-libtmux-0.8.2/lib/python2.7/site-packages/libtmux/session.py", line 134, in switch_client
    raise exc.LibTmuxException(proc.stderr)
libtmux.exc.LibTmuxException: [u'no current client']

Start in tmuxp/cli.py at load_workspace and _reattach, then inspect libtmux session.switch_client handling. Reproduce the issue with TMUX= tmuxp load ... and compare it with env -u TMUX tmuxp load .... Done means attaching to an existing session succeeds without the “no current client” traceback.