Integration tests and xhr

Hi *,
I have a page that displays a partial containing a form (_followup) that uses ajax to post to a controller action 'add_followup' that adds some elements to a list.
I'm not able to post to that form in integration tests.
What I'm using now is

     xml_http_request "issue/add_followup",
       :email => foo.email, :username => foo.username, :subject => "[ISSUE #1] foo",
       :body => "This is a test followup.", :issue_id => issue.id

I really have no clue on how to handle this. The response is 500, using the console I discovered it's a NoMethodError.
Suggestions ?

   ngw