can't yield from root fiber (FiberError)

[bendodd]

I'm trying to implement Sinatra-synchrony on a Sinatra app, which is tested using Rspec and Selenium, but when I run the tests I get this error:

  1) Public donation using cr theme with javascript Browser capabilities
     Failure/Error: Unable to find matching line from backtrace
     FiberError:
       can't yield from root fiber
     # ./spec/features/spec_helper.rb:61:in `delete_cookies'
     # ./spec/features/spec_helper.rb:31:in `block (2 levels) in <top (required)>'

I have ENV['RACK_ENV'] = 'test', but I can't workout what else to change to allow the tests to run correctly.

Thanks in advance, Ben