diff --git a/src/components/__tests__/calendar-default-alert-ids.test.ts b/src/components/__tests__/calendar-default-alert-ids.test.ts
new file mode 100644
index 00000000..bc56bad6
--- /dev/null
+++ b/src/components/__tests__/calendar-default-alert-ids.test.ts
@@ -0,0 +1,30 @@
+import { describe, it, expect, vi } from 'vitest';
+
+// The calendar sheet's default reminders. Alert ids must be unique in the
+// account on Stalwart 1.0: an alert the lists can't show kept the id
+// another client gave it, and when another calendar used that id too, the
+// whole Calendar/set was refused, the new name and colour with it.
+
+vi.mock('../icons', () => ({ Check: () => null, Plus: () => null, X: () => null }));
+vi.mock('../Button', () => ({ default: () => null }));
+vi.mock('../../lib/use-layout', () => ({ useSheetInsets: () => ({}) }));
+vi.mock('../../theme/colors', () => ({ useColors: () => ({}) }));
+
+import type { Alert } from '../../api/types';
+import { defaultAlertsToSave } from '../calendar/CalendarEditSheet';
+
+// An email alert: the reminder lists show display alerts only.
+const email: Alert = { '@type': 'Alert', trigger: { '@type': 'OffsetTrigger', offset: '-PT30M', relativeTo: 'start' }, action: 'email' };
+
+describe('the default alerts a calendar saves', () => {
+  it('puts the alerts it keeps under the calendar\'s own ids too', () => {
+    const timed = defaultAlertsToSave({ id: 'acc-2:b', originalId: 'b' }, false, [{ minutesBefore: 10 }], { 'default-1': email });
+    const allDay = defaultAlertsToSave({ id: 'b' }, true, [], { x1: email });
+
+    expect(timed).toEqual({
+      'cal-b-timed-1': email,
+      'cal-b-timed-2': expect.objectContaining({ action: 'display' }),
+    });
+    expect(allDay).toEqual({ 'cal-b-allday-1': email });
+  });
+});
diff --git a/src/components/calendar/CalendarEditSheet.tsx b/src/components/calendar/CalendarEditSheet.tsx
index 60b65a0b..eb435147 100644
--- a/src/components/calendar/CalendarEditSheet.tsx
+++ b/src/components/calendar/CalendarEditSheet.tsx
@@ -110,6 +110,27 @@ function DefaultRemindersEditor({
   );
 }
 
+/**
+ * The default alerts saved for `calendar`'s timed or all-day events: the
+ * reminders, and the alerts the lists can't show (`kept`), every one under
+ * the calendar's own id prefix. Ids must be unique in the account, and a kept
+ * alert had the id another client gave it: when another calendar used that
+ * id too, Stalwart 1.0 refused the whole Calendar/set, its name and colour
+ * with it.
+ */
+export function defaultAlertsToSave(
+  calendar: Pick<Calendar, 'id' | 'originalId'>,
+  allDay: boolean,
+  reminders: Reminder[],
+  kept: Record<string, Alert>,
+): Record<string, Alert> {
+  const prefix = defaultAlertIdPrefix(calendar, allDay);
+  const rekeyed: Record<string, Alert> = {};
+  let n = 0;
+  for (const alert of Object.values(kept)) rekeyed[`${prefix}-${++n}`] = alert;
+  return remindersToDefaultAlerts(reminders, rekeyed, prefix);
+}
+
 interface CalendarEditSheetProps {
   visible: boolean;
   // Existing calendar to edit; null creates a new one.
@@ -161,10 +182,10 @@ export function CalendarEditSheet({ visible, calendar, onSave, onClose }: Calend
       const values: CalendarEditValues = { name: trimmed, color, description: description.trim() };
       // Ids unique in the account: Stalwart 1.0 refuses one another calendar uses.
       if (calendar && editsDefaults && !sameReminders(timedReminders, initialTimed)) {
-        values.defaultAlertsWithTime = remindersToDefaultAlerts(timedReminders, keptTimed, defaultAlertIdPrefix(calendar, false));
+        values.defaultAlertsWithTime = defaultAlertsToSave(calendar, false, timedReminders, keptTimed);
       }
       if (calendar && editsDefaults && !sameReminders(allDayReminders, initialAllDay)) {
-        values.defaultAlertsWithoutTime = remindersToDefaultAlerts(allDayReminders, keptAllDay, defaultAlertIdPrefix(calendar, true));
+        values.defaultAlertsWithoutTime = defaultAlertsToSave(calendar, true, allDayReminders, keptAllDay);
       }
       await onSave(values);
       onClose();