diff --git a/mydefstruct.lisp b/mydefstruct.lisp
index f703319..dbc6f69 100644
--- a/mydefstruct.lisp
+++ b/mydefstruct.lisp
@@ -1,3 +1,8 @@
+(defpackage :my-structures
+  (:use :cl)
+  (:export #:mydefstruct))
+
+(in-package :my-structures)
 
 (defun constructor-name (sym)
   (intern (concatenate 'string "MAKE-" (string sym))))
@@ -8,19 +13,31 @@
 (defun setter-name (name sym)
   (intern (concatenate 'string "SET-" (string name) "-" (string sym))))
 
+(defun obj-type (obj)
+  (car obj))
+
+(defun make-error-message (real expected)
+  (format nil "Accessor called on wrong type! Expected ~a but found ~a"
+	  expected real))
+
+
 (defun constructor (name slots)
   `(defun ,(constructor-name name) ,slots
-     (list ,@slots)))
+     (list ',name ,@slots)))
 (defun accessors (name slots)
   (loop for slot in slots
-	for i upfrom 0 collect
+	for i upfrom 1 collect
 	`(defun ,(accessor-name name slot) (obj)
-	   (nth ,i obj))))
+	   (if (eql (obj-type obj) ',name)
+	       (nth ,i obj)
+	       (error (make-error-message (obj-type obj) ',name))))))
 (defun setters (name slots)
   (loop for slot in slots
-	for i upfrom 0 collect
+	for i upfrom 1 collect
 	`(defun ,(setter-name name slot) (obj val)
-	   (setf (nth ,i obj) val))))
+	   (if (eql (obj-type obj) ',name)
+	       (setf (nth ,i obj) val)
+	       (error (make-error-message (obj-type obj) ',name))))))
 (defun setfers (name slots)
   (loop for slot in slots collect
 	`(defsetf ,(accessor-name name slot)