• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: macOS 11.1 Big Sur
• Poetry version: 1.1.5
• Link of a Gist with the contents of your pyproject.toml file: https://gist.github.com/nikmolnar/cc95e80d0cf6c9256b53294b759b3881

Issue

I've discovered a bug that arises when pytest is combined with Python ^2.7 in pyproject.toml. The Gist linked above provides a minimal example of this issue. Running poetry lock with the above file results in the following exception:

AttributeError

  'EmptyConstraint' object has no attribute 'min'

The error does not appear if either pytest is removed or the Python version is changed to ^3.6. I originally reported this as part of #2372, but then realized that that issue related to error handing in response to an error in pyproject.toml. As this error occurs in a correctly-formatted pyproject.toml, I believe it is a distinct issue.