diff --git a/src/hb-ot-hdmx-table.hh b/src/hb-ot-hdmx-table.hh
index f30d3c3e2..cbcf6f5f2 100644
--- a/src/hb-ot-hdmx-table.hh
+++ b/src/hb-ot-hdmx-table.hh
@@ -94,7 +94,7 @@ struct hdmx
   bool serialize (hb_serialize_context_t *c,
 		  unsigned version,
 		  Iterator it,
-		  const hb_vector_t<hb_codepoint_pair_t> new_to_old_gid_list,
+		  const hb_vector_t<hb_codepoint_pair_t> &new_to_old_gid_list,
 		  unsigned num_glyphs)
   {
     TRACE_SERIALIZE (this);