diff --git a/racket/src/racket/src/mzmarksrc.c b/racket/src/racket/src/mzmarksrc.c
index a4bf998059..c2c222894c 100644
--- a/racket/src/racket/src/mzmarksrc.c
+++ b/racket/src/racket/src/mzmarksrc.c
@@ -1592,7 +1592,8 @@ place_async_channel_val {
   gcMARK2(pac->msg_chains, gc);
   gcMARK2(pac->wakeup_signal, gc);
 
-  /* mark master-allocated objects within each messages: */
+  /* mark master-allocated objects within each messages; the
+     raw pairs that form the list are embedded in each message block */
   j = pac->out;
   sz = pac->size;
   for (i = pac->count; i--; ) {
diff --git a/racket/src/racket/src/place.c b/racket/src/racket/src/place.c
index e60f5baa5c..f76094ae6d 100644
--- a/racket/src/racket/src/place.c
+++ b/racket/src/racket/src/place.c
@@ -3012,11 +3012,12 @@ Scheme_Place_Async_Channel *place_async_channel_create() {
 #endif
 
   ch = GC_master_malloc_tagged(sizeof(Scheme_Place_Async_Channel));
+  ch->so.type = scheme_place_async_channel_type;
+
   msgs = GC_master_malloc(sizeof(Scheme_Object*) * 8);
   msg_memory = GC_master_malloc(sizeof(void*) * 8);
   msg_chains = GC_master_malloc(sizeof(Scheme_Object*) * 8);
 
-  ch->so.type = scheme_place_async_channel_type;
   ch->in = 0;
   ch->out = 0;
   ch->count = 0;
@@ -3186,6 +3187,8 @@ static void place_async_send(Scheme_Place_Async_Channel *ch, Scheme_Object *uo)
   int cnt;
 
   o = places_serialize(uo, &msg_memory, &master_chain, &invalid_object);
+  /* uo needs to stay live until `master_chain` is registered in `ch` */
+
   if (!o) {
     if (invalid_object) {
       scheme_contract_error("place-channel-put",
@@ -3245,6 +3248,9 @@ static void place_async_send(Scheme_Place_Async_Channel *ch, Scheme_Object *uo)
     maybe_report_message_size(ch);
   }
 
+  /* make sure `uo` is treated as live until here: */
+  if (!uo) scheme_signal_error("?");
+
   {
     intptr_t msg_size;
     msg_size = GC_message_allocator_size(msg_memory);
diff --git a/racket/src/racket/src/schpriv.h b/racket/src/racket/src/schpriv.h
index be48d6a773..b534d466b8 100644
--- a/racket/src/racket/src/schpriv.h
+++ b/racket/src/racket/src/schpriv.h
@@ -4114,7 +4114,7 @@ typedef struct Scheme_Place_Async_Channel {
 #endif
   Scheme_Object **msgs;
   void **msg_memory;
-  Scheme_Object **msg_chains;
+  Scheme_Object **msg_chains; /* lists embedded in message blocks; specially traversed during GC */
   intptr_t mem_size;
   intptr_t reported_size; /* size reported to master GC; avoid reporting too often */
   void *wakeup_signal;