diff --git a/specs/simple-serialize.md b/specs/simple-serialize.md
index 4c6d33652..2adff2388 100644
--- a/specs/simple-serialize.md
+++ b/specs/simple-serialize.md
@@ -13,9 +13,11 @@
         - [Composite types](#composite-types)
         - [Aliases](#aliases)
         - [Default values](#default-values)
+        - [Illegal types](#illegal-types)
     - [Serialization](#serialization)
         - [`"uintN"`](#uintn)
         - [`"bool"`](#bool)
+        - [`"null`](#null)
         - [Vectors, containers, lists, unions](#vectors-containers-lists-unions)
     - [Deserialization](#deserialization)
     - [Merkleization](#merkleization)