diff --git a/src/class.c b/src/class.c
index 8afd792f5..180d66c1a 100644
--- a/src/class.c
+++ b/src/class.c
@@ -392,7 +392,6 @@ prepare_singleton_class(mrb_state *mrb, struct RBasic *o)
   }
   o->c = sc;
   mrb_field_write_barrier(mrb, (struct RBasic*)o, (struct RBasic*)sc);
-  mrb_field_write_barrier(mrb, (struct RBasic*)sc, (struct RBasic*)o);
   mrb_obj_iv_set(mrb, (struct RObject*)sc, MRB_SYM(__attached__), mrb_obj_value(o));
   sc->flags |= o->flags & MRB_FL_OBJ_IS_FROZEN;
 }