--format=dir crashes at the end of the build

[hynek]

While I was able to build an archive artifact, the very same situation with an added --format=dir explodes with an:

Build artifact successfully created.
  Artifact: None
Traceback (most recent call last):
  File "/Users/hynek/.virtualenvs/platter/bin/platter", line 9, in <module>
    load_entry_point('platter==1.0.dev0', 'console_scripts', 'platter')()
  File "/Users/hynek/.virtualenvs/platter/lib/python2.7/site-packages/click/core.py", line 610, in __call__
    return self.main(*args, **kwargs)
  File "/Users/hynek/.virtualenvs/platter/lib/python2.7/site-packages/click/core.py", line 590, in main
    rv = self.invoke(ctx)
  File "/Users/hynek/.virtualenvs/platter/lib/python2.7/site-packages/click/core.py", line 936, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/hynek/.virtualenvs/platter/lib/python2.7/site-packages/click/core.py", line 782, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/hynek/.virtualenvs/platter/lib/python2.7/site-packages/click/core.py", line 416, in invoke
    return callback(*args, **kwargs)
  File "/Users/hynek/.virtualenvs/platter/src/platter/platter.py", line 669, in build_cmd
    postbuild_script=postbuild_script)
  File "/Users/hynek/.virtualenvs/platter/src/platter/platter.py", line 564, in build
    self.finalize(artifact, time.time() - now)
  File "/Users/hynek/.virtualenvs/platter/src/platter/platter.py", line 506, in finalize
    if not os.path.isfile(artifact):
  File "/Users/hynek/.virtualenvs/platter/lib/python2.7/genericpath.py", line 29, in isfile
    st = os.stat(path)
TypeError: coercing to Unicode: need string or buffer, NoneType found

Since it seems to happen after the build, the directory with the artifact is present within dist. When building an archive, I get

Build artifact successfully created.
  Artifact: dist/domains_ws-0-macosx-10.10-x86_64.tar.gz
  MD5: 362367c55544ba61a834470dd7a31a75
  SHA1: ccf93002b9c24c27d19d80b27f8a7edde4e1d520