Create a broken form with http.Client

How to create a 'broken' form for testing purposes with http.Client that will trigger a response error on ParseForm() in a handler function?

I have the following code:

func (app *App) signupUser(w http.ResponseWriter, r *http.Request) {
    err := r.ParseForm()
    if err != nil { // how to trigger this if statement?
        sendStatus(w, http.StatusBadRequest, "error parsing form")
        return
    }
...

Then during testing I'm using http.Client to test this controller:

resp, err = client.PostForm(ts.URL+"/user/signup", url.Values{
            "email":      {testEmail},
            "password":   {goodPassword},
)
if err != nil { 
  t.Error(err) // I never get here
}