diff --git a/racket/collects/racket/math.rkt b/racket/collects/racket/math.rkt
index f22a9b8ba1..df28590218 100644
--- a/racket/collects/racket/math.rkt
+++ b/racket/collects/racket/math.rkt
@@ -47,7 +47,8 @@
   (define (nonnegative-integer? x)
     (and (integer? x) (not (negative? x))))
 
-  (define natural? exact-nonnegative-integer?)
+  (define (natural? x)
+    (exact-nonnegative-integer? x))
 
   ;; z^2
   (define (sqr z)