diff --git a/typed-racket-lib/typed-racket/base-env/prims.rkt b/typed-racket-lib/typed-racket/base-env/prims.rkt
index da4921b3..0ac4028b 100644
--- a/typed-racket-lib/typed-racket/base-env/prims.rkt
+++ b/typed-racket-lib/typed-racket/base-env/prims.rkt
@@ -808,7 +808,13 @@ the typed racket language.
                    (define i 0)
                    (for (clauses ...)
                      (define v body-expr)
-                     (cond [(unsafe-fx= i 0)  (define new-vs (ann (make-vector n v) T))
+                     ;; can't use `unsafe-fx=` here
+                     ;; if `n` is larger than a fixnum, this is unsafe, and we
+                     ;; don't know whether that's the case until we try creating
+                     ;; the vector
+                     ;; other unsafe ops are after vector allocation, and so are
+                     ;; fine
+                     (cond [(= i 0)  (define new-vs (ann (make-vector n v) T))
                                               (set! vs new-vs)]
                            [else  (unsafe-vector-set! vs i v)])
                      (set! i (unsafe-fx+ i 1))