diff --git a/src/hb-open-type.hh b/src/hb-open-type.hh
index 7dea171df..5f06b96ec 100644
--- a/src/hb-open-type.hh
+++ b/src/hb-open-type.hh
@@ -554,7 +554,7 @@ struct ArrayOf
     return_trace (true);
   }
   template <typename Iterator>
-    hb_enable_if_t (hb_is_iterator (Iterator, Type),
+    hb_enable_if_t (hb_is_iterator (Iterator, const Type),
   bool) serialize (hb_serialize_context_t *c,
 		   Iterator items)
   {