diff --git a/pkgs/compatibility-pkgs/compatibility-lib/mzlib/unit200.rkt b/pkgs/compatibility-pkgs/compatibility-lib/mzlib/unit200.rkt
index d1dd8ec..29f4968 100644
--- a/pkgs/compatibility-pkgs/compatibility-lib/mzlib/unit200.rkt
+++ b/pkgs/compatibility-pkgs/compatibility-lib/mzlib/unit200.rkt
@@ -7,14 +7,13 @@
 		      syntax/name
 		      syntax/context
                       racket/syntax
+                      racket/undefined
 		      (only racket/base filter)
 		      "private/unitidmap.rkt")
 
   ;; ----------------------------------------------------------------------
   ;; Structures and helpers
 
-  (define undefined (letrec ([x x]) x)) ; initial value
-
   (define insp (current-inspector)) ; for named structures
 
   (define-struct unit (num-imports exports go)) ; unit value