diff --git a/src/hb-ot-hmtx-table.hh b/src/hb-ot-hmtx-table.hh
index 4c7dabedc..d5e1fc91d 100644
--- a/src/hb-ot-hmtx-table.hh
+++ b/src/hb-ot-hmtx-table.hh
@@ -170,13 +170,12 @@ struct hmtxvmtx
   {
     friend struct hmtxvmtx;
 
-    accelerator_t (hb_face_t *face,
-		   unsigned int default_advance_ = 0)
+    accelerator_t (hb_face_t *face)
     {
       table = hb_sanitize_context_t ().reference_table<hmtxvmtx> (face, T::tableTag);
       var_table = hb_sanitize_context_t ().reference_table<HVARVVAR> (face, T::variationsTag);
 
-      default_advance = default_advance_ ? default_advance_ : hb_face_get_upem (face);
+      default_advance = T::is_horizontal ? hb_face_get_upem (face) / 2 : hb_face_get_upem (face);
 
       /* Populate count variables and sort them out as we go */