diff --git a/typed-racket-lib/typed-racket/rep/type-mask.rkt b/typed-racket-lib/typed-racket/rep/type-mask.rkt
index 57696d19..e62ade6b 100644
--- a/typed-racket-lib/typed-racket/rep/type-mask.rkt
+++ b/typed-racket-lib/typed-racket/rep/type-mask.rkt
@@ -11,7 +11,7 @@
 ;;
 ;; - - Details - -
 ;;
-;; Type masks are represented with a simple 31-bit fixnum.
+;; Type masks are represented with a simple 30-bit fixnum.
 ;;
 ;; If a bit flag in a Type's bitmask is set to 1, it means the Type
 ;; _may_ overlap with the values described by that bit flag.
@@ -51,10 +51,13 @@
 ;; type mask predicate
 (define-syntax type-mask? (make-rename-transformer #'fixnum?))
 
-;; define the max size of type  masks
+;; define the max size of type  masks -- if we limit the size to
+;; 30 we are guaranteed to be a fixnum on 32 and 64 bit machines.
+;; (30 is the max number of bits available in a 2's complement 
+;; tagged integer on a 32-bit machine)
 (module const racket/base
   (provide max-mask-size)
-  (define max-mask-size 31))
+  (define max-mask-size 30))
 (require 'const (for-syntax 'const))