It was working fine before, but not anymore:

+ poetry install
(...)
[RuntimeError]
The Poetry configuration is invalid:
  - {...} is not of type 'object'

I have compared the pip install poetry outputs of both working and non-working versions. The only difference is tomlkit 0.5.8 -> 0.5.9

I have changed my CI command line to do pip install poetry "tomlkit=0.5.8" and it's working fine again.

Let me know if you need more information about my project setup.