diff --git a/collects/tests/racket/optimize.rktl b/collects/tests/racket/optimize.rktl
index cb4db4eac8..bf2e847d66 100644
--- a/collects/tests/racket/optimize.rktl
+++ b/collects/tests/racket/optimize.rktl
@@ -860,6 +860,39 @@
            '(lambda () (box 0.0))
            #f)
 
+;; Make sure that a mutable top-level isn't copy-propagated
+;; across another effect:
+(test-comp '(module m racket/base
+              (define x 10)
+              (define (f y)
+                (let ([old x])
+                  (set! x y)
+                  (set! x old))))
+           '(module m racket/base
+              (define x 10)
+              (define (f y)
+                (let ([old x])
+                   (set! x y)
+                   (set! x x))))
+            #f)
+
+;; Do copy-propagate a reference to a mutable top-level 
+;; across non-effects:
+(test-comp '(module m racket/base
+              (define x 10)
+              (define (f y)
+                (let ([old x])
+                  (list (cons y y)
+                        (set! x old)))))
+           '(module m racket/base
+              (define x 10)
+              (define (f y)
+                (begin
+                  x ; compiler might not determine that `x' is definitely defined
+                  (list (cons y y)
+                        (set! x x)))))
+            #f)
+
 (test-comp '(let ([x 1][y 2]) x)
 	   '1)
 (test-comp '(let ([x 1][y 2]) (+ y x))
diff --git a/src/racket/src/optimize.c b/src/racket/src/optimize.c
index 511df50a46..28094ffb70 100644
--- a/src/racket/src/optimize.c
+++ b/src/racket/src/optimize.c
@@ -488,7 +488,7 @@ static int single_valued_noncm_expression(Scheme_Object *expr, int fuel)
   Scheme_Object *rator = NULL;
 
  switch (SCHEME_TYPE(expr)) {
- case scheme_toplevel_type:
+ case scheme_compiled_toplevel_type:
    return 1;
  case scheme_application_type:
    rator = ((Scheme_App_Rec *)expr)->args[0];
@@ -2718,12 +2718,12 @@ set_optimize(Scheme_Object *data, Optimize_Info *info, int context)
     delta = optimize_info_get_shift(info, pos);
     if (delta)
       var = scheme_make_local(scheme_local_type, pos + delta, 0);
-
-    info->vclock++;
   } else {
     optimize_info_used_top(info);
   }
 
+  info->vclock++;
+
   sb->var = var;
   sb->val = val;
 
@@ -3156,7 +3156,7 @@ int scheme_compiled_propagate_ok(Scheme_Object *value, Optimize_Info *info)
 
 
   if (SAME_TYPE(SCHEME_TYPE(value), scheme_compiled_toplevel_type)) {
-    if (SCHEME_TOPLEVEL_FLAGS(value) >= SCHEME_TOPLEVEL_FIXED)
+    if ((SCHEME_TOPLEVEL_FLAGS(value) & SCHEME_TOPLEVEL_FLAGS_MASK) >= SCHEME_TOPLEVEL_FIXED)
       return 1;
     if (info->top_level_consts) {
       int pos;