Rake "already initialized constant WFKV_" warning

Trying to run rake cucumber:ok and am getting this error:

/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rack-1.3.4/lib/rack/backports/uri/common_192.rb:53: warning: already initialized constant WFKV_

Then:
Command failed with status (1): [/Users/dev/.rbenv/versions/1.9.2-p290/bin...]

I am pretty new to Rails and Google didn't turn anything up for this error.

EDIT: I've tried adding bundle exec and that makes no difference.

Here's what I got with --trace:

/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/file_utils.rb:53:in `block in create_shell_runner'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/file_utils.rb:45:in `call'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/file_utils.rb:45:in `sh'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/file_utils_ext.rb:36:in `sh'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/cucumber-1.1.0/lib/cucumber/rake/task.rb:104:in `run'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/cucumber-1.1.0/lib/cucumber/rake/task.rb:193:in `block in define_task'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/task.rb:205:in `call'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/task.rb:205:in `block in execute'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/task.rb:200:in `each'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/task.rb:200:in `execute'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/task.rb:158:in `block in invoke_with_call_chain'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/1.9.1/monitor.rb:201:in `mon_synchronize'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/task.rb:151:in `invoke_with_call_chain'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/task.rb:144:in `invoke'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/application.rb:112:in `invoke_task'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/application.rb:90:in `block (2 levels) in top_level'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/application.rb:90:in `each'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/application.rb:90:in `block in top_level'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/application.rb:129:in `standard_exception_handling'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/application.rb:84:in `top_level'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/application.rb:62:in `block in run'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/application.rb:129:in `standard_exception_handling'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/lib/rake/application.rb:59:in `run'
/Users/dev/.rbenv/versions/1.9.2-p290/lib/ruby/gems/1.9.1/gems/rake-0.9.2/bin/rake:32:in `<top (required)>'
/Users/dev/.rbenv/versions/1.9.2-p290/bin/rake:19:in `load'
/Users/dev/.rbenv/versions/1.9.2-p290/bin/rake:19:in `<main>'
Tasks: TOP => cucumber:ok

I started having the same problem this evening. It seems to be related to Rack 1.3.4. I fixed it by adding this to my Gemfile:

gem 'rack', '1.3.3'

Then running:

bundle update rack

Incidentally, I tried Bozhidar's suggestion before this, but to no avail.

Rack 1.3.5 is out now, which has fixed this warning.