diff --git a/collects/tests/mzscheme/sync.ss b/collects/tests/mzscheme/sync.ss
index afd819bc25..1216bdacef 100644
--- a/collects/tests/mzscheme/sync.ss
+++ b/collects/tests/mzscheme/sync.ss
@@ -745,6 +745,9 @@
                         (thread-send t 'x (lambda () (values 'a 'z)))))
   (err/rt-test (thread-send t 'x)))
 
+;; make sure it's ok for rewind to be the first action:
+(test (void) thread-wait (thread (lambda () (thread-rewind-receive '(1 2 3)))))
+
 ;; ----------------------------------------
 ;;  Garbage collection
 
diff --git a/src/mzscheme/src/sema.c b/src/mzscheme/src/sema.c
index 5388d570f4..f68a76f28d 100644
--- a/src/mzscheme/src/sema.c
+++ b/src/mzscheme/src/sema.c
@@ -1053,6 +1053,8 @@ static void mbox_push_front(Scheme_Thread *p, Scheme_Object *lst)
   int cnt = -1;
   Scheme_Object *next, *hd;
 
+  make_mbox_sema(p);
+
   next = lst;
   while (!SCHEME_NULLP(next)) {
     /* Push one: */
@@ -1065,12 +1067,10 @@ static void mbox_push_front(Scheme_Thread *p, Scheme_Object *lst)
     next = SCHEME_CDR(next);
 
     if (SCHEME_NULLP(next) || (cnt == 256)) {
-      /* Either done or need to use fuel */
-      if (cnt > -1) {
-        /* do a single post for all messages */
-        ((Scheme_Sema*)p->mbox_sema)->value += cnt;
-        scheme_post_sema(p->mbox_sema);
-      }
+      /* Either done or need to pause to allow breaks/swaps; */
+      /* do a single post for all messages so far. */
+      ((Scheme_Sema*)p->mbox_sema)->value += cnt;
+      scheme_post_sema(p->mbox_sema);
       SCHEME_USE_FUEL(cnt+1); /* might sleep */
       cnt = -1;
     }