diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index e8ba019..69c8a10 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -333,10 +333,9 @@ rb_mmtk_call_obj_free(MMTk_ObjectReference object)
 
     rb_gc_obj_free(objspace, obj);
 
-    // TODO: uncomment this when done debugging
-// #ifdef MMTK_DEBUG
+#ifdef MMTK_DEBUG
     memset((void *)obj, 0, rb_gc_impl_obj_slot_size(obj));
-// #endif
+#endif
 }
 
 static size_t