diff --git a/src/hb-ot-shape-complex-arabic.cc b/src/hb-ot-shape-complex-arabic.cc
index ea2a68fcc..e9db00545 100644
--- a/src/hb-ot-shape-complex-arabic.cc
+++ b/src/hb-ot-shape-complex-arabic.cc
@@ -81,6 +81,7 @@ static unsigned int get_joining_type (hb_codepoint_t u, hb_unicode_general_categ
   if (unlikely (hb_in_range<hb_codepoint_t> (u, 0xA840, 0xA872)))
   {
       if (unlikely (u == 0xA872))
+	/* XXX Looks like this should be TYPE_L, but we don't support that yet! */
 	return JOINING_TYPE_R;
 
       return JOINING_TYPE_D;