diff --git a/src/hb-machinery-private.hh b/src/hb-machinery-private.hh
index 119625c45..39a7fd1f6 100644
--- a/src/hb-machinery-private.hh
+++ b/src/hb-machinery-private.hh
@@ -639,7 +639,8 @@ struct hb_lazy_loader_t
 
   inline void set_stored (Stored *instance_)
   {
-    assert (instance == nullptr);
+    if (instance)
+      thiz ()->destroy (instance);
     instance = instance_;
   }