diff --git a/server/src/nest/assistant/assistant-undo.helpers.ts b/server/src/nest/assistant/assistant-undo.helpers.ts
index 266d3b8f6..ecaf06ce3 100644
--- a/server/src/nest/assistant/assistant-undo.helpers.ts
+++ b/server/src/nest/assistant/assistant-undo.helpers.ts
@@ -72,6 +72,9 @@ const CHECKS: Readonly<Partial<Record<string, Check>>> = {
   ),
 };
 
+/** The call a check reads when its caller has none: no arguments, no held card. */
+const NO_CALL: UndoCall = { args: {}, preview: null };
+
 /**
  * True when the change `toolName` made (its stored `result`) is still as it
  * left it. `call` is what the answer may not name (the day a vacation toggle
@@ -81,7 +84,7 @@ export function unchangedSinceApplied(
   toolName: string,
   result: unknown,
   snapshot: TripSnapshot,
-  call: UndoCall = { args: {}, preview: null },
+  call: UndoCall = NO_CALL,
 ): boolean {
   const check = CHECKS[toolName];
   return !!check && check(recordOf(result), { ...snapshot, reads: snapshot.reads ?? new Map() }, call);
diff --git a/server/src/nest/assistant/tool-call-preview.helpers.ts b/server/src/nest/assistant/tool-call-preview.helpers.ts
index f37c01648..b134ea08f 100644
--- a/server/src/nest/assistant/tool-call-preview.helpers.ts
+++ b/server/src/nest/assistant/tool-call-preview.helpers.ts
@@ -379,6 +379,9 @@ function bookedDay(result: unknown): number | null {
   return reservation && typeof reservation === 'object' ? id(reservation as Args, 'day_id') : null;
 }
 
+/** Where a change came from when its caller does not say: no held card, no trip. */
+const NOT_FROM: AppliedFrom = { preview: null, tripId: null };
+
 /**
  * The receipt of an applied change: what it touched (the created row from its
  * answer, the changed one from its arguments), the day "Show on map" goes to
@@ -393,7 +396,7 @@ export function appliedChange(
   result: unknown,
   dayId: number | null,
   undoUntil: string | null,
-  from: AppliedFrom = { preview: null, tripId: null },
+  from: AppliedFrom = NOT_FROM,
 ): AssistantApplied | null {
   // A tool without a card of its own touched something the client has no view for.
   const runs = writeToolOf(toolName, args);