diff --git a/web/src/store/__tests__/recurrence.test.ts b/web/src/store/__tests__/recurrence.test.ts
index d48a955..f6e0ae2 100644
--- a/web/src/store/__tests__/recurrence.test.ts
+++ b/web/src/store/__tests__/recurrence.test.ts
@@ -15,9 +15,9 @@ describe("isRecurring", () => {
   it("does not call a one-off event a series just because it has a baseEventId", () => {
     expect(isRecurring(ev({ baseEventId: "ev1" }))).toBe(false);
     expect(isRecurring(ev({}))).toBe(false);
-  });
-  it("still recognises an occurrence whose base is another event", () => {
-    expect(isRecurring(ev({ id: "ev1_2", baseEventId: "ev1" }))).toBe(true);
+    // The shape a live 0.16.19 returns for a one-off: an instance id of its own,
+    // and a base that is a different id. Neither makes it a series.
+    expect(isRecurring(ev({ id: "eaaaaai", baseEventId: "i" }))).toBe(false);
   });
   it("recognises a series by its recurrence rules", () => {
     expect(isRecurring(ev({ recurrenceRules: [{ "@type": "RecurrenceRule", frequency: "weekly" }] }))).toBe(true);
diff --git a/web/src/store/calendar.ts b/web/src/store/calendar.ts
index d74967b..04d2b8f 100644
--- a/web/src/store/calendar.ts
+++ b/web/src/store/calendar.ts
@@ -300,15 +300,15 @@ export const useCalendar = create<CalendarState>((set, get) => ({
 /**
  * Whether an event is part of a series.
  *
- * Not the same question as "does it have a baseEventId": `CalendarEvent/query`
- * runs with `expandRecurrences`, and Stalwart sets `baseEventId` on every event
- * it returns that way — a one-off event included, pointing at itself. Recurrence
- * rules are what make a series, so those are the question; a base that is some
- * *other* event means this is one occurrence of one, whether or not the expanded
- * instance carried the rules along with it.
+ * Not the same question as "does it have a baseEventId", nor "is that base some
+ * other event". `CalendarEvent/query` runs with `expandRecurrences`, and Stalwart
+ * hands back an instance id for everything it returns that way — a one-off event
+ * included, whose own id (`eaaaaai`) differs from its base (`i`), verified
+ * against a live 0.16.19. Recurrence rules are what make a series, so those are
+ * what we ask about.
  */
 export function isRecurring(ev: CalendarEvent): boolean {
-  return Boolean(ev.recurrenceRules?.length || ev.excludedRecurrenceRules?.length || (ev.baseEventId && ev.baseEventId !== ev.id));
+  return Boolean(ev.recurrenceRules?.length || ev.excludedRecurrenceRules?.length);
 }
 
 export function toInstance(e: CalendarEvent, calendars: Record<Id, Calendar>): EventInstance | null {