diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index e0331cd65..c43b5bc79 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc
@@ -263,7 +263,8 @@ hb_insert_dotted_circle (hb_buffer_t *buffer, hb_font_t *font)
 static void
 hb_form_clusters (hb_buffer_t *buffer)
 {
-  if (buffer->cluster_level != HB_BUFFER_CLUSTER_LEVEL_MONOTONE_GRAPHEMES)
+  if (!(buffer->scratch_flags & HB_BUFFER_SCRATCH_FLAG_HAS_NON_ASCII) ||
+      buffer->cluster_level != HB_BUFFER_CLUSTER_LEVEL_MONOTONE_GRAPHEMES)
     return;
 
   /* Loop duplicated in hb_ensure_native_direction(). */