poetry: Error on `poetry update`

• 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: Ubuntu 18.04

• Poetry version: 0.12.2

• Link of a Gist with the contents of your pyproject.toml file:

Issue

When running poetry update I hit the following error:

$ poetry update
Updating dependencies
Resolving dependencies... (4.0s)
                                                 
[AttributeError]                  
'NoneType' object has no attribute 'requires'  
                                                 
update [--no-dev] [--dry-run] [--lock] [--] [<packages>]...

This only happens with a git-based dependency specified in my pyproject.toml, in the following form:

results = { git = "ssh://git@github.com/whatever/whatever.git", branch = "master" }