diff --git a/src/mzscheme/src/fun.c b/src/mzscheme/src/fun.c
index 45da74c0ce..f18478e1a5 100644
--- a/src/mzscheme/src/fun.c
+++ b/src/mzscheme/src/fun.c
@@ -6624,8 +6624,8 @@ static Scheme_Object *continuation_marks(Scheme_Thread *p,
   Scheme_Cont_Mark_Set *set;
   Scheme_Object *cache, *nt;
   long findpos, bottom;
-  long cmpos, cdelta = 0;
-  int found_tag = 0;
+  long cmpos, first_cmpos = 0, cdelta = 0;
+  int found_tag = 0, at_mc_boundary = 0;
 
   if (cont && SAME_OBJ(cont->prompt_tag, prompt_tag))
     found_tag = 1;
@@ -6651,6 +6651,7 @@ static Scheme_Object *continuation_marks(Scheme_Thread *p,
       cmpos = (long)mc->cont_mark_pos;
       cdelta = mc->cont_mark_offset;
       bottom = 0;
+      at_mc_boundary = 1;
     } else {
       findpos = (long)MZ_CONT_MARK_STACK;
       cmpos = (long)MZ_CONT_MARK_POS;
@@ -6736,10 +6737,16 @@ static Scheme_Object *continuation_marks(Scheme_Thread *p,
 
       if (cache) {
         if (((Scheme_Cont_Mark_Chain *)cache)->key) {
-          if (last)
+          if (last) {
             last->next = (Scheme_Cont_Mark_Chain *)cache;
-          else
+            if (at_mc_boundary) {
+              SCHEME_MARK_CHAIN_FLAG(last) |= 0x1;
+              at_mc_boundary = 0;
+            }
+          } else {
             first = (Scheme_Cont_Mark_Chain *)cache;
+            first_cmpos = cmpos;
+          }
           
           found_tag = 1; /* cached => tag is there */
         } else {
@@ -6756,7 +6763,7 @@ static Scheme_Object *continuation_marks(Scheme_Thread *p,
       } else {
         Scheme_Cont_Mark_Chain *pr;
         pr = MALLOC_ONE_RT(Scheme_Cont_Mark_Chain);
-        pr->so.type = scheme_cont_mark_chain_type;
+        pr->iso.so.type = scheme_cont_mark_chain_type;
         pr->key = find[pos].key;
         pr->val = find[pos].val;
         pr->pos = find[pos].pos;
@@ -6822,10 +6829,16 @@ static Scheme_Object *continuation_marks(Scheme_Thread *p,
                           (Scheme_Object *)pr);
           find[pos].cache = cache;
         }
-        if (last)
+        if (last) {
           last->next = pr;
-        else
+          if (at_mc_boundary) {
+            SCHEME_MARK_CHAIN_FLAG(last) |= 1;
+            at_mc_boundary = 0;
+          }
+        } else {
           first = pr;
+          first_cmpos = cmpos;
+        }
 
         last = pr;
       }
@@ -6876,10 +6889,13 @@ static Scheme_Object *continuation_marks(Scheme_Thread *p,
   nt = NULL;
 #endif
 
+  if (first && (first_cmpos < first->pos))
+    scheme_signal_error("internal error: bad mark-stack position");
+
   set = MALLOC_ONE_TAGGED(Scheme_Cont_Mark_Set);
   set->so.type = scheme_cont_mark_set_type;
   set->chain = first;
-  set->cmpos = cmpos;
+  set->cmpos = first_cmpos;
   set->native_stack_trace = nt;
 
   return (Scheme_Object *)set;
@@ -7099,10 +7115,12 @@ extract_cc_markses(int argc, Scheme_Object *argv[])
   prompt_tag = SCHEME_PTR_VAL(prompt_tag);
 
   chain = ((Scheme_Cont_Mark_Set *)argv[0])->chain;
-  last_pos = ((Scheme_Cont_Mark_Set *)argv[0])->cmpos + 2;
+  last_pos = -1;
 
   while (chain) {
     for (i = 0; i < len; i++) {
+      if (SCHEME_MARK_CHAIN_FLAG(chain) & 0x1)
+        last_pos = -1;
       if (SAME_OBJ(chain->key, keys[i])) {
 	long pos;
 	pos = (long)chain->pos;
diff --git a/src/mzscheme/src/schpriv.h b/src/mzscheme/src/schpriv.h
index 1b5e1fa9f3..840c0f4d3e 100644
--- a/src/mzscheme/src/schpriv.h
+++ b/src/mzscheme/src/schpriv.h
@@ -1149,13 +1149,15 @@ typedef struct Scheme_Cont_Mark {
 } Scheme_Cont_Mark;
 
 typedef struct Scheme_Cont_Mark_Chain {
-  Scheme_Object so;
+  Scheme_Inclhash_Object iso; /* 0x1 => next is from different meta-continuation */
   Scheme_Object *key;
   Scheme_Object *val;
   MZ_MARK_POS_TYPE pos;
   struct Scheme_Cont_Mark_Chain *next;
 } Scheme_Cont_Mark_Chain;
 
+#define SCHEME_MARK_CHAIN_FLAG(c) MZ_OPT_HASH_KEY(&(c)->iso)
+
 typedef struct Scheme_Cont_Mark_Set {
   Scheme_Object so;
   struct Scheme_Cont_Mark_Chain *chain;