diff --git a/racket/src/cs/compile-file.ss b/racket/src/cs/compile-file.ss
index e0cae03187..bb0bb0d2ec 100644
--- a/racket/src/cs/compile-file.ss
+++ b/racket/src/cs/compile-file.ss
@@ -2,12 +2,15 @@
 ;; Check to make we're using a build of Chez Scheme
 ;; that has all the features we need.
 
-(define (check-defined expr)
-  (unless (guard (x [else #f]) (eval expr))
+(define (check-ok what thunk)
+  (unless (guard (x [else #f]) (thunk))
     (error 'compile-file
            (format
             "failed trying `~a`; probably you need a newer Chez Scheme"
-            expr))))
+            what))))
+
+(define (check-defined expr)
+  (check-ok expr (lambda () (eval expr))))
 
 (check-defined 'box-cas!)
 (check-defined 'make-arity-wrapper-procedure)
@@ -19,6 +22,16 @@
 (check-defined '(define-ftype T (function __collect_safe () void)))
 (check-defined 'call-setting-continuation-attachment)
 (check-defined 'hashtable-cells)
+(check-ok "fxvector-set!"
+          (lambda ()
+            (parameterize ([optimize-level 3]
+                           [run-cp0 (lambda (cp0 x) x)])
+
+              (eval '(define (op x)
+                       (if (fx- 0) 0 0)))
+              (eval '(define (f x)
+                       (fxvector-set! x 0 (op 0))))
+              (eval '(f (fxvector 0))))))
 
 ;; ----------------------------------------
 
diff --git a/racket/src/io/common/fixnum.rkt b/racket/src/io/common/fixnum.rkt
index 187f967f4a..8c6178318f 100644
--- a/racket/src/io/common/fixnum.rkt
+++ b/racket/src/io/common/fixnum.rkt
@@ -7,7 +7,6 @@
 ;; Representing a mutable, fixnum-valued variable with an fxvector can
 ;; avoid a write barrier on assignment
 
-#;
 (define-syntax-rule (define-fixnum id v)
   (begin
     (define cell (fxvector v))
@@ -18,8 +17,3 @@
            [(set! _ r) #'(fxvector-set! cell 0 r)]
            [(... (_ ...)) (raise-syntax-error stx "bad use" stx)]
            [_ #'(fxvector-ref cell 0)]))))))
-
-;; ... but, for now, something seems to go wrong with whole-program
-;; optimization
-(define-syntax-rule (define-fixnum id v)
-  (define id v))