diff --git a/src/formulare/core.clj b/src/formulare/core.clj
index 9f4dd93..2a50f9a 100644
--- a/src/formulare/core.clj
+++ b/src/formulare/core.clj
@@ -122,7 +122,7 @@
   (str (hash [def values])))
 
 (defn render-widgets [form-def values req]
-  {:pre [(spec/assert ::form form-def)]}
+  {:pre [(spec/valid? ::form form-def)]}
   (let [form-hash (form-hash form-def values)
         submitted-hash (get-in req [:params :__form-hash])
         validate? (= form-hash submitted-hash)]
@@ -141,7 +141,7 @@
           (anti-forgery-field))))
 
 (defn form-data [form-def req]
-  {:pre [(spec/assert ::form form-def)]}
+  {:pre [(spec/valid? ::form form-def)]}
   (reduce (fn [coll [id field]]
             (let [value (get-in req [:params id])]
               (assoc coll