I have barebones rails 3.2.9 app with mongoid 3.0.14. I am using a pristine mongoid.yml straight from the generator. When I try to access any models, I get the error message:

Problem:
  No configuration could be found for a session named ''.
Summary:
  When attempting to create the new session, Mongoid could not find a session configuration for the name: ''. This is necessary in order to know the host, port, and options needed to connect.

I have confirmed that my Rails ENV is properly set to development and that the yaml file is being parsed correctly. Using the debugger, I have traced the issue to the session method in sessions.rb. For some inexplicable reason, despite session_override returning nil (which it should), the first code path is still executed, and Sessions.with_name(nil) is called, instead of Sessions.default. See this gist:
https://gist.github.com/4148114

This issue occurs when I am not using the debugger, so I don't think its a heisenbug. Any help is appreciated.