diff --git a/src/send.rs b/src/send.rs
index 2606434..0ca62fd 100644
--- a/src/send.rs
+++ b/src/send.rs
@@ -28,6 +28,7 @@ use crate::stream::*;
 //   - if error:
 //       - u8: error kind, encoded using error::io_errorkind_to_u8
 //       - rest: error message
+//   - absent for cancel message
 
 pub(crate) type RequestID = u32;
 pub(crate) type ChunkLength = u16;