Your missing route looks like some auto-generated assets path with nil input. Are you sure your assets are correctly specified in your css/sass or wherever this route comes from?
I have been struggling with a similiar issue where an asset path induced a routing error
1) user signup: [ JS ] : creates User on successful signup
Failure/Error: Unable to find matching line from backtrace
ActionController::RoutingError:
No route matches [GET] "/assets/images/leftArrow.png"
followed by a stack trace virtually identical to yours. In my case it turned out the asset was in fact missing (which went unnoticed in my development.log and in my test environment until recently I recklessly 'bundle updated' an project after several idle months)
Along the way I learned a lot about Capybara's app_host and asset handling, assets precompiling in different environments, Application.configure.assets.[debug|digest|...] and alike, which probably is where you should be searching in the first place. Or in the sprockets/sass-rails url helper ... after all your missing route still looks like some auto-generated assets path with empty input)
If the above doesn't help a workaround might be adding the following line in environments/test.rb:
config.action_dispatch.show_exceptions = true
While not addressing the problem directly, it successfully suppressed it in my case.
Another kind of workaround could be in spec_helper.rb:
ActionController::Base.asset_host = "http://myapp.dev"
where myapp.dev is a running instance of myApp in development mode, which delivers the assets or at least does not hit your test routing for the assets, but probably one should do this only in complete desparation. It's inspired by the strategy to avoid asset compiling as explained in
http://johnbintz.github.com/blog/2012/01/07/cucumber-capybara-asset-pipeline-debug-mode/
Also potentially helpful:
http://guides.rubyonrails.org/asset_pipeline.html
http://rubydoc.info/github/jnicklas/capybara/master/Capybara#configure-class_method
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