how can I get the stack trace for a 500 error to show up in test.log?

I'm driving a rails 3.0.10 app with capaybara/seleinium and when I get a 500 error I can't get the stack trace to appear in log/test.log

All I see is the following:

   Rendered resource_pages/show.html.haml within layouts/application (15.2ms)
   Completed 500 Internal Server Error in 250ms

When I put a debugger statement in the view I found what was causing the error:

   nil.html_safe

and tracing a bit further I find @original_exception.backtrace with all the info I want displayed ... but it doesn't get rendered to the log.