rcov not detecting request.xhr? test

I have a method

def index
  if request.xhr?
    @users = User.find( :all )

And then I have a test for it:

def test_index
  user = users( :foo )
  xhr :post, :index, {}, { :user_id => user.user_id }
  assert_response :success
  assert_template 'users/index'

The test works fine. If I put a puts in there it fires, so I know the
test is hitting the block in question.

So then when I run rcov, it produces a report that says the code block
has not yet been covered with a test.

I also tried xhr :get in my test but got the same result.

Anyone know anything about this? Is there a workaround, or another way?