When a providers callback is called, it can result in no access token being generated, resulting in a call to expired? on nil.
This error is generated in /lib/omniauth/strategies/oauth2.rb:92
In the instances I'm seeing, this triggered by a get request to the google-oauth2/callback with no query string
omniauth-google-oauth2 (0.8.0)
omniauth-oauth2 (1.7.0)