diff --git a/routing/package.lisp b/routing/package.lisp
index 708ab2b..76868b0 100644
--- a/routing/package.lisp
+++ b/routing/package.lisp
@@ -55,6 +55,7 @@
    #:route-path
    #:with-request-params
    #:with-response-data
-   #:with-request-data))
+   #:with-request-data
+   #:not-implemented))
 
 (in-package :wst.routing)
diff --git a/routing/responses.lisp b/routing/responses.lisp
index 6ca6e84..bded327 100644
--- a/routing/responses.lisp
+++ b/routing/responses.lisp
@@ -66,3 +66,9 @@
  CONTENT is any object that is serialized accourding to the type.")
   (:method ((ty t) response &key)
     (write-response response :status 422 :content "unprocessable entity")))
+
+(defgeneric not-implemented (ty response &key)
+  (:documentation "Build a response for a type TY (:json, :html, t = html).
+ CONTENT is any object that is serialized accourding to the type.")
+  (:method ((ty t) response &key)
+    (write-response response :status 501 :content "not implemented")))