diff --git a/src/hb-buffer-private.h b/src/hb-buffer-private.h
index 7ed68062f..2d2ea0629 100644
--- a/src/hb-buffer-private.h
+++ b/src/hb-buffer-private.h
@@ -75,7 +75,7 @@ struct _hb_buffer_t {
   unsigned int in_length;
   unsigned int out_length;
   unsigned int in_pos;
-  unsigned int out_pos;
+  unsigned int out_pos; /* out_length and out_pos are actually always the same */
 
   hb_internal_glyph_info_t     *in_string;
   hb_internal_glyph_info_t     *out_string;