diff --git a/todo.md b/todo.md
index ea80ae6..5bb8c55 100644
--- a/todo.md
+++ b/todo.md
@@ -201,7 +201,8 @@ and is also what the panel ranked hardest — two methods, one answer.
       `tagValue` is `TlvValue`, so `{ callback_num: { tagValue: buf } }` compiles and is refused
       only at runtime, and reading `receipted_message_id` has to narrow out arrays it can never
       hold. Derive the types from the specs' own wire types and `multiple` flag. It has to land in
-      the same minor as the arrays, or narrowing the types is a second break. From the
+      the same minor as the arrays, or narrowing the types is a second break. A test compiles the
+      CHANGELOG's `tagValue[0]` advice, which the union does not type-check today. From the
       product-owner review of #25.
 
 - [ ] **Settle what a repeated tag not marked `multiple` reads as, and pin it in a test.** A vendor