java.lang.ClassCastException: clojure.lang.Var$Unbound cannot be cast to clojure.lang.IDeref

(defn error-item [[first-error]]
  [:span.error first-error])

(defn form-html [username password verify email]
  (h/html
    [:h2 "Form validation"]
    (form-to [:post "/signup"]
      (label "user" "Username")
      (text-field "username" username)
      (vali/on-error :username error-item)
      [:br]