diff --git a/gc.c b/gc.c
index 793ab4a8..28192de2 100644
--- a/gc.c
+++ b/gc.c
@@ -786,6 +786,18 @@ void gc_stack_mark_refs_gray(gc_thread_data *thd, object obj, int depth)
   }
 }
 
+/**
+ * Determine if object lives on the thread's stack
+ */
+int gc_is_stack_obj(gc_thread_data *thd, object obj)
+{
+  char tmp;
+  object low_limit = &tmp;
+  object high_limit = thd->stack_start;
+  return (check_overflow(low_limit, obj) && 
+          check_overflow(obj, high_limit));
+}
+
 /**
 Write barrier for updates to heap-allocated objects
 Plans:
@@ -804,8 +816,15 @@ void gc_mut_update(gc_thread_data *thd, object old_obj, object value)
 ////fprintf(stderr, " for heap object ");
 //fprintf(stderr, "\n");
     gc_mark_gray(thd, old_obj);
-    // TODO: need this too???
-    gc_stack_mark_gray(thd, value);
+// TODO: check if value is on the heap,
+//   if so, mark gray right now
+//   otherwise set it to be marked after moved to heap during next GC
+    //gc_stack_mark_gray(thd, value);
+    if (gc_is_stack_obj(thd, value)) {
+      grayed(value) = 1;
+    } else {
+      gc_mark_gray(thd, value);
+    }
   } else if (stage == STAGE_TRACING) {
 //fprintf(stderr, "DEBUG - GC async tracing marking heap obj %p ", old_obj);
 //Cyc_display(old_obj, stderr);
diff --git a/include/cyclone/types.h b/include/cyclone/types.h
index 7b74f290..3b6c85b6 100644
--- a/include/cyclone/types.h
+++ b/include/cyclone/types.h
@@ -95,11 +95,11 @@ struct gc_heap_t {
 
 typedef struct gc_header_type_t gc_header_type;
 struct gc_header_type_t {
-  //unsigned char mark; // mark bits (only need 2)
   unsigned int mark; // mark bits (only need 2)
-  // TODO: forwarding address (probably not needed for mark/sweep), anything else???
+  unsigned char grayed; // stack object to be grayed when moved to heap
 };
 #define mark(x) (((list) x)->hdr.mark)
+#define grayed(x) (((list) x)->hdr.grayed)
 
 /* HEAP definitions */
 // experimenting with a heap based off of the one in Chibi scheme
@@ -158,6 +158,7 @@ void gc_thr_add_to_move_buffer(gc_thread_data *d, int *alloci, object obj);
 void gc_thread_data_init(gc_thread_data *thd, int mut_num, char *stack_base, long stack_size);
 void gc_thread_data_free(gc_thread_data *thd);
 // Prototypes for mutator/collector:
+int gc_is_stack_obj(gc_thread_data *thd, object obj);
 void gc_mut_update(gc_thread_data *thd, object old_obj, object value);
 void gc_mut_cooperate(gc_thread_data *thd, int buf_len);
 void gc_stack_mark_refs_gray(gc_thread_data *thd, object obj, int depth);