diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index 7a84d9b2c..a9a8a7a3d 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc
@@ -202,8 +202,10 @@ hb_form_clusters (hb_ot_shape_context_t *c)
 {
   unsigned int count = c->buffer->len;
   for (unsigned int i = 1; i < count; i++)
-    /* TODO: Match other mark types too? */
-    if (c->buffer->info[i].general_category() == HB_UNICODE_GENERAL_CATEGORY_NON_SPACING_MARK)
+    if (FLAG (c->buffer->info[i].general_category()) &
+	(FLAG (HB_UNICODE_GENERAL_CATEGORY_SPACING_MARK) |
+	 FLAG (HB_UNICODE_GENERAL_CATEGORY_ENCLOSING_MARK) |
+	 FLAG (HB_UNICODE_GENERAL_CATEGORY_NON_SPACING_MARK)))
       c->buffer->info[i].cluster = c->buffer->info[i - 1].cluster;
 }