diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index 3a13bb59c..60b028d34 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc
@@ -188,7 +188,7 @@ hb_form_clusters (hb_buffer_t *buffer)
 	(FLAG (HB_UNICODE_GENERAL_CATEGORY_SPACING_MARK) |
 	 FLAG (HB_UNICODE_GENERAL_CATEGORY_ENCLOSING_MARK) |
 	 FLAG (HB_UNICODE_GENERAL_CATEGORY_NON_SPACING_MARK)))
-      buffer->info[i].cluster = buffer->info[i - 1].cluster; /* XXX do the min() here */
+      buffer->merge_clusters (i - 1, i + 1);
 }
 
 static void
@@ -405,6 +405,8 @@ hb_ot_shape_execute_internal (hb_ot_shape_context_t *c)
   HB_BUFFER_ALLOCATE_VAR (c->buffer, unicode_props0);
   HB_BUFFER_ALLOCATE_VAR (c->buffer, unicode_props1);
 
+  c->buffer->clear_output ();
+
   hb_set_unicode_props (c->buffer);
 
   hb_form_clusters (c->buffer);