diff --git a/lib/includes/spdylay/spdylay.h b/lib/includes/spdylay/spdylay.h
index e0e60f4..bb28a57 100644
--- a/lib/includes/spdylay/spdylay.h
+++ b/lib/includes/spdylay/spdylay.h
@@ -984,6 +984,11 @@ typedef struct {
  * |callbacks|. |user_data| is an arbitrary user supplied data, which
  * will be passed to the callback functions.
  *
+ * Some of the members of |callbacks| can be ``NULL``, but
+ * :member:`spdylay_session_callbacks.send_callback` and
+ * :member:`spdylay_session_callbacks.recv_callback` are must be
+ * specified.
+ *
  * This function returns 0 if it succeeds, or one of the following
  * negative error codes:
  *
@@ -1008,6 +1013,11 @@ int spdylay_session_client_new(spdylay_session **session_ptr,
  * |callbacks|. |user_data| is an arbitrary user supplied data, which
  * will be passed to the callback functions.
  *
+ * Some of the members of |callbacks| can be ``NULL``, but
+ * :member:`spdylay_session_callbacks.send_callback` and
+ * :member:`spdylay_session_callbacks.recv_callback` are must be
+ * specified.
+ *
  * This function returns 0 if it succeeds, or one of the following
  * negative error codes:
  *