Gtk3: ValueError in _split_environ_list() on start

[cheese]

git branch next commit 40ff9e3
Fedora 27 x86_64

$ ./zim.py -D
Traceback (most recent call last):
  File "./zim.py", line 39, in <module>
    import zim
  File "/run/media/cheese/cheesenotes/upstream/zim-desktop-wiki/zim/__init__.py", line 195, in <module>
    import zim.config
  File "/run/media/cheese/cheesenotes/upstream/zim-desktop-wiki/zim/config/__init__.py", line 9, in <module>
    from .basedirs import *
  File "/run/media/cheese/cheesenotes/upstream/zim-desktop-wiki/zim/config/basedirs.py", line 89, in <module>
    set_basedirs()
  File "/run/media/cheese/cheesenotes/upstream/zim-desktop-wiki/zim/config/basedirs.py", line 82, in set_basedirs
    _split_environ_list(os.environ.get('XDG_CONFIG_DIRS', ('/etc/xdg/',)))))
  File "/run/media/cheese/cheesenotes/upstream/zim-desktop-wiki/zim/config/basedirs.py", line 23, in _split_environ_list
    raise ValueError
ValueError