diff --git a/contract/tools/ui-kit-goals/marshalTables.js b/contract/tools/ui-kit-goals/marshalTables.js
index 1d734e79..82220bd4 100644
--- a/contract/tools/ui-kit-goals/marshalTables.js
+++ b/contract/tools/ui-kit-goals/marshalTables.js
@@ -9,6 +9,12 @@
 // @ts-check
 import { Far, makeMarshal } from '@endo/marshal';
 
+/**
+ * The null slot indicates that identity is not intended to be preserved.
+ *
+ * @typedef { string | null } OptionalSlot
+ */
+
 /**
  * Implement conventional parts of convertValToSlot, convertSlotToVal functions
  * for use with makeMarshal based on a slot <-> value translation table,
@@ -17,7 +23,7 @@ import { Far, makeMarshal } from '@endo/marshal';
  *
  * @template Val
  * @param {(val: Val, size: number) => string} makeSlot
- * @param {(slot: string | null, iface: string | undefined) => Val} makeVal
+ * @param {(slot: OptionalSlot, iface: string | undefined) => Val} makeVal
  */
 const makeTranslationTable = (makeSlot, makeVal) => {
   /** @type {Map<Val, string>} */
@@ -37,7 +43,7 @@ const makeTranslationTable = (makeSlot, makeVal) => {
     return slot;
   };
 
-  /** @type {(slot: string | null, iface: string | undefined) => Val} */
+  /** @type {(slot: OptionalSlot, iface: string | undefined) => Val} */
   const convertSlotToVal = (slot, iface) => {
     if (slot === null) return makeVal(slot, iface);
     if (slotToVal.has(slot)) {
@@ -53,7 +59,7 @@ const makeTranslationTable = (makeSlot, makeVal) => {
   return harden({ convertValToSlot, convertSlotToVal });
 };
 
-/** @type {(slot: string | null, iface: string | undefined) => any} */
+/** @type {(slot: OptionalSlot, iface: string | undefined) => any} */
 const synthesizeRemotable = (slot, iface) =>
   Far(`${(iface ?? '').replace(/^Alleged: /, '')}#${slot}`, {});