[sdispater]

Author of Poetry here!

I've never seen that error before.

Which version of Python do you use?

And feel free to create an issue on the issue tracker: https://github.com/sdispater/poetry/issues

[sametmax]

pyproject debates aside, I love your work. I think poetry is a beautiful piece of software, the source code is very easy to read and the 2/3 compat is well done (although I would not use assert to check things).