diff --git a/pkgs/racket-test-core/tests/racket/optimize.rktl b/pkgs/racket-test-core/tests/racket/optimize.rktl
index f9d9d35278..ef985a25ca 100644
--- a/pkgs/racket-test-core/tests/racket/optimize.rktl
+++ b/pkgs/racket-test-core/tests/racket/optimize.rktl
@@ -1026,6 +1026,15 @@
                       (set! x x))))
             #f)
 
+;; Don't move a side-effecting experssion past an unsafe operation
+;; that observes effects:
+(test-comp '(lambda (b f)
+             (let* ([x (f (lambda () b))])
+               (cons (unsafe-unbox b) x)))
+           '(lambda (b f)
+             (cons (unsafe-unbox b) (f (lambda () b))))
+           #f)
+
 (test-comp '(module m racket/base
              (define (true) #t)
              (define no (if (true) (lambda (x) (cons x 'no)) display))
diff --git a/racket/src/racket/src/optimize.c b/racket/src/racket/src/optimize.c
index 1dc433dd45..fd26ba58be 100644
--- a/racket/src/racket/src/optimize.c
+++ b/racket/src/racket/src/optimize.c
@@ -72,7 +72,8 @@ struct Optimize_Info
   /* Propagated up and down the chain: */
   int size;
   int vclock; /* virtual clock that ticks for a side effect, a branch,
-                 or a dependency on an earlier side-effect (such as a
+                 observation of a side effect (such as an unbox),
+                 or a dependency on an earlier side effect (such as a
                  previous guard on an unsafe operation's argument);
                  the clock is only compared between binding sites and
                  uses, so we can rewind the clock at a join after an
@@ -2983,7 +2984,8 @@ static int is_nonmutating_nondependant_primitive(Scheme_Object *rator, int n)
    unsafe operation is defined */
 {
   if (SCHEME_PRIMP(rator)
-      && (SCHEME_PRIM_PROC_OPT_FLAGS(rator) & (SCHEME_PRIM_IS_OMITABLE | SCHEME_PRIM_IS_OMITABLE_ALLOCATION))
+      && ((SCHEME_PRIM_PROC_OPT_FLAGS(rator) & (SCHEME_PRIM_IS_OMITABLE | SCHEME_PRIM_IS_OMITABLE_ALLOCATION))
+          && !(SCHEME_PRIM_PROC_OPT_FLAGS(rator) & (SCHEME_PRIM_IS_UNSAFE_OMITABLE)))
       && (n >= ((Scheme_Primitive_Proc *)rator)->mina)
       && (n <= ((Scheme_Primitive_Proc *)rator)->mu.maxa))
     return 1;
diff --git a/racket/src/racket/src/schpriv.h b/racket/src/racket/src/schpriv.h
index 9121f4f180..16e765b0c8 100644
--- a/racket/src/racket/src/schpriv.h
+++ b/racket/src/racket/src/schpriv.h
@@ -81,7 +81,8 @@
 #define SCHEME_PRIM_IS_NARY_INLINED        (1 << 2)
 /* indicates that a primitive call can be dropped if it's result is not used;
    although the function never raises an exception, it should not be reordered
-   past a test that might be a guard: */
+   past a test that might be a guard or past an expression that might
+   have a side effect: */
 #define SCHEME_PRIM_IS_UNSAFE_OMITABLE     (1 << 3)
 /* indicates that a primitive call can be dropped if it's result is not used,
    because it has no side-effect and never raises an exception: */