diff --git a/src/graph/gsubgpos-graph.hh b/src/graph/gsubgpos-graph.hh
index dd99e09d9..beabec9f0 100644
--- a/src/graph/gsubgpos-graph.hh
+++ b/src/graph/gsubgpos-graph.hh
@@ -39,7 +39,7 @@ struct make_extension_context_t
 {
   hb_tag_t table_tag;
   graph_t& graph;
-  hb_vector_t<char> buffer;
+  hb_vector_t<char>& buffer;
   hb_hashmap_t<unsigned, graph::Lookup*> lookups;
 
   HB_INTERNAL make_extension_context_t (hb_tag_t table_tag_,