I wanted to use al:show-native-message-box

which is equivalent of

(defcfun ("al_show_native_message_box" show-native-message-box) :int
  (display :pointer) (title :string) (heading :string)
  (text :string) (buttons :string) (flags messagebox-flags))

But when i wrote:

(in-package #:al-tut)
 
(defvar *created-display*)

(defun main ()
  (unless (al:init)
    (al:show-native-message-box (c:null-pointer) (c:null-pointer) (c:null-pointer) "allegro init failed." (c:null-pointer) (c:null-pointer))
    'AL-INIT-FAILED)
  (setf *created-display* (al:create-display 800 600))
  (unless *created-display*
    (al:show-native-message-box (c:null-pointer) (c:null-pointer) (c:null-pointer) "window init failed. " (c:null-pointer) (c:null-pointer))
    'AL-CREATE-DISPLAY-FAILED)
  (c:with-foreign-string (text "MessageBox Title") 
    (al:show-native-message-box *created-display* 
				text
				(c:foreign-enum-value 'al::messagebox-flags :error)
				(c:foreign-enum-value 'al::filechooser-modes :show-hidden)
				(c:null-pointer)
				(c:null-pointer)))
  (al:destroy-display *created-display*))

where

(defcenum messagebox-flags
  (:warn 1)
  (:error 2)
  (:ok-cancel 4)
  (:yes-no 8)
  (:question 16))

in the source code.

But SBCL gives me the error:

2 is not a Lisp string or pointer.
   [Condition of type SIMPLE-ERROR]

Need help. Thanks