diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index 5386e36..d2d17a5 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -304,7 +304,7 @@
 			    unsigned int end,
 			    char *buf,
 			    unsigned int buf_size,
-			    unsigned int *buf_consumed,
+			    unsigned int *buf_consumed, /* May be NULL */
 			    hb_font_t *font, /* May be NULL */
 			    hb_buffer_serialize_format_t format,
 			    hb_buffer_serialize_flags_t flags);
@@ -313,7 +313,7 @@
 hb_buffer_deserialize_glyphs (hb_buffer_t *buffer,
 			      const char *buf,
 			      unsigned int buf_len,
-			      unsigned int *buf_consumed,
+			      unsigned int *buf_consumed, /* May be NULL */
 			      hb_font_t *font, /* May be NULL */
 			      hb_buffer_serialize_format_t format);