diff --git a/src/class.c b/src/class.c
index 8751dcaf0..0c15fbf42 100644
--- a/src/class.c
+++ b/src/class.c
@@ -1376,6 +1376,7 @@ boot_defclass(mrb_state *mrb, struct RClass *super)
   if (super) {
     c->super = super;
     mrb_field_write_barrier(mrb, (struct RBasic*)c, (struct RBasic*)super);
+    c->flags |= MRB_FL_CLASS_IS_INHERITED;
   }
   else {
     c->super = mrb->object_class;