diff --git a/racket/src/racket/src/fun.c b/racket/src/racket/src/fun.c
index 3161ca589d..99eea00330 100644
--- a/racket/src/racket/src/fun.c
+++ b/racket/src/racket/src/fun.c
@@ -4786,7 +4786,8 @@ static void copy_in_runstack(Scheme_Thread *p, Scheme_Saved_Stack *isaved, int s
 static void copy_in_mark_stack(Scheme_Thread *p, Scheme_Cont_Mark *cont_mark_stack_copied,
 			       MZ_MARK_STACK_TYPE cms, MZ_MARK_STACK_TYPE base_cms,
 			       intptr_t copied_offset, Scheme_Object **_sub_conts,
-                               int clear_caches)
+                               int clear_caches,
+                               MZ_MARK_POS_TYPE new_mark_pos)
      /* Copies in the mark stack up to depth cms, but assumes that the
 	stack up to depth base_cms is already in place (probably in
 	place for a dynamic-wind context in an continuation
@@ -4829,6 +4830,10 @@ static void copy_in_mark_stack(Scheme_Thread *p, Scheme_Cont_Mark *cont_mark_sta
     }
   }
 
+  /* Updated after potential GC: */
+  MZ_CONT_MARK_POS = new_mark_pos;
+  MZ_CONT_MARK_STACK = cms;
+
   if (_sub_conts) {
     if (*_sub_conts) {
       sub_cont = (Scheme_Cont *)SCHEME_CAR(*_sub_conts);
@@ -5322,12 +5327,11 @@ static MZ_MARK_STACK_TYPE exec_dyn_wind_pres(Scheme_Dynamic_Wind_List *dwl,
            dynamic-wind context. Clear cached info on restore
            if there's a prompt. */
         DW_PrePost_Proc pre = dwl->dw->pre;
-        MZ_CONT_MARK_POS = dwl->dw->envss.cont_mark_pos;
-        MZ_CONT_MARK_STACK = dwl->dw->envss.cont_mark_stack;
         copy_in_mark_stack(p, cont->cont_mark_stack_copied, 
-                           MZ_CONT_MARK_STACK, copied_cms,
+                           dwl->dw->envss.cont_mark_stack, copied_cms,
                            cont->cont_mark_offset, _sub_conts,
-                           clear_cm_caches);
+                           clear_cm_caches,
+                           dwl->dw->envss.cont_mark_pos);
         copied_cms = MZ_CONT_MARK_STACK;
 
         if (!skip_dws)
@@ -5931,12 +5935,11 @@ static void restore_continuation(Scheme_Cont *cont, Scheme_Thread *p, int for_pr
 
   /* Finish copying cont mark stack back in. */
     
-  MZ_CONT_MARK_POS = cont->ss.cont_mark_pos;
-  MZ_CONT_MARK_STACK = cont->ss.cont_mark_stack;
   copy_in_mark_stack(p, cont->cont_mark_stack_copied, 
-                     MZ_CONT_MARK_STACK, copied_cms,
+                     cont->ss.cont_mark_stack, copied_cms,
                      cont->cont_mark_offset, &sub_conts,
-                     clear_cm_caches);
+                     clear_cm_caches,
+                     cont->ss.cont_mark_pos);
         
   if (SAME_OBJ(result, SCHEME_MULTIPLE_VALUES)) {
     p->ku.multiple.array = mv;
@@ -6328,7 +6331,7 @@ void scheme_takeover_stacks(Scheme_Thread *p)
       op->cont_mark_stack_swapped = swapped;
     }
     *(p->cont_mark_stack_owner) = p;
-    copy_in_mark_stack(p, p->cont_mark_stack_swapped, MZ_CONT_MARK_STACK, 0, 0, NULL, 0);
+    copy_in_mark_stack(p, p->cont_mark_stack_swapped, MZ_CONT_MARK_STACK, 0, 0, NULL, 0, MZ_CONT_MARK_POS);
     p->cont_mark_stack_swapped = NULL;
   }
 }
diff --git a/racket/src/racket/src/thread.c b/racket/src/racket/src/thread.c
index 69d3eadafb..55b7a318e8 100644
--- a/racket/src/racket/src/thread.c
+++ b/racket/src/racket/src/thread.c
@@ -9199,7 +9199,7 @@ static void prepare_thread_for_GC(Scheme_Object *t)
       
     /* zero unused part of last mark stack segment */
     segpos = ((intptr_t)p->cont_mark_stack >> SCHEME_LOG_MARK_SEGMENT_SIZE);
-    
+
     if (segpos < p->cont_mark_seg_count) {
       Scheme_Cont_Mark *seg = p->cont_mark_stack_segments[segpos];
       int stackpos = ((intptr_t)p->cont_mark_stack & SCHEME_MARK_SEGMENT_MASK);