Why is my OpenId authentication failing?

Question

I'm porting my OpenId authentication scheme from Sinatra to Rails 2.3.8.

Everything works well up until I get the response back from my OpenId provider. The Response object I get back is an OpenId::Consumer::FailureResponse, when it should be a SuccessResponse.

I ported this directly from Sinatra, and the only difference is what session object I'm using. (Rails session now vs. Rack session with Sinatra.) I know that things are working on the Provider end (not under my control).

The offending code looks something like this:

# POST /login/openid
def begin_login
  openid = URI.parse(params[:openid]).to_s

  begin
    request = openid_consumer.begin(openid)
  rescue OpenID::DiscoveryFailure => message
    "Sorry, we couldn't find your identifier '#{openid}'"
  else
    # root_url and complete_login_url are defined in my routes file
    # and the latter points to the next method: complete_login
    redirect_to request.redirect_url(root_url, complete_login_url)
  end  
end

# POST /login/openid/complete
def complete_login
  openid_consumer ||= OpenID::Consumer.new(session,
    OpenID::Store::Filesystem.new("#{RAILS_ROOT}/tmp/openid"))

  response = openid_consumer.complete(params, request.url)
  response.class # => OpenID::Consumer::FailureResponse
end