diff --git a/src/hb-aat-layout-common.hh b/src/hb-aat-layout-common.hh
index fa34afaa7..02ee09058 100644
--- a/src/hb-aat-layout-common.hh
+++ b/src/hb-aat-layout-common.hh
@@ -98,7 +98,10 @@ struct hb_aat_apply_context_t :
     using_buffer_glyph_set = buffer->len >= 4;
 
     if (using_buffer_glyph_set)
+    {
+      buffer_glyph_set.clear ();
       buffer->collect_codepoints (buffer_glyph_set);
+    }
   }
   bool buffer_intersects_machine () const
   {