diff --git a/typed-racket-lib/typed-racket/base-env/base-env.rkt b/typed-racket-lib/typed-racket/base-env/base-env.rkt
index 58b198f5..3168cad5 100644
--- a/typed-racket-lib/typed-racket/base-env/base-env.rkt
+++ b/typed-racket-lib/typed-racket/base-env/base-env.rkt
@@ -2918,6 +2918,7 @@
 ;; Section 15.8
 [system-type
  (cl->*
+  (-> (Un (-val 'unix) (-val 'windows) (-val 'macosx)))
   (-> (-val 'os) (Un (-val 'unix) (-val 'windows) (-val 'macosx)))
   (-> (-val 'gc) (Un (-val 'cgc) (-val '3m)))
   (-> (-val 'link) (Un (-val 'static) (-val 'shared) (-val 'dll) (-val 'framework)))