diff --git a/vm.h b/vm.h
index e814f1e..f37f724 100644
--- a/vm.h
+++ b/vm.h
@@ -13,6 +13,7 @@ enum garbageData {
 struct garbageObject {
   enum garbageData type;
   struct garbageObject *next;
+  unsigned char mark;
 
   union {
     int value;