diff --git a/admin/src/settings/SettingsScreen.test.tsx b/admin/src/settings/SettingsScreen.test.tsx
index 51c6784..94a0609 100644
--- a/admin/src/settings/SettingsScreen.test.tsx
+++ b/admin/src/settings/SettingsScreen.test.tsx
@@ -432,14 +432,18 @@ describe("SettingsScreen — preferences", () => {
 
   it("a 422 shows the API detail naming the field and keeps the edit", async () => {
     const { form, onClose } = await renderPreferences();
-    fireEvent.change(field(form, "Planning horizon (days)"), { target: { value: "0" } });
-    writeResponse = () => jsonResponse({ detail: "horizon_days must be >= 1" }, 422);
+    fireEvent.change(field(form, "Notify target"), { target: { value: "notify.nobody" } });
+    writeResponse = () => jsonResponse({ detail: "notify_target is not a notify service" }, 422);
     save(form);
     const message = await screen.findByTestId("preferences-message");
-    expect(message.textContent).toBe("The preferences could not be saved: horizon_days must be >= 1");
+    expect(message.textContent).toBe(
+      "The preferences could not be saved: notify_target is not a notify service",
+    );
     expect(message.getAttribute("role")).toBe("alert");
-    expect(field(form, "Planning horizon (days)").value).toBe("0");
-    expect(writes()).toEqual([{ method: "PATCH", path: SETTINGS_PATH, body: { horizon_days: 0 } }]);
+    expect(field(form, "Notify target").value).toBe("notify.nobody");
+    expect(writes()).toEqual([
+      { method: "PATCH", path: SETTINGS_PATH, body: { notify_target: "notify.nobody" } },
+    ]);
     expect(onClose).not.toHaveBeenCalled();
   });
 
@@ -453,12 +457,17 @@ describe("SettingsScreen — preferences", () => {
     expect(checkbox(form, "Celebration").checked).toBe(true);
   });
 
-  it("a malformed horizon is rejected before any request", async () => {
+  it.each([
+    ["empty", ""],
+    ["zero", "0"],
+    ["oversized (Infinity would serialize as null and reset the field)", "9".repeat(400)],
+    ["past the safe-integer range", "123456789012345678901"],
+  ])("a %s horizon is rejected before any request", async (_case, value) => {
     const { form } = await renderPreferences();
-    fireEvent.change(field(form, "Planning horizon (days)"), { target: { value: "" } });
+    fireEvent.change(field(form, "Planning horizon (days)"), { target: { value } });
     save(form);
     expect(screen.getByTestId("preferences-message").textContent).toBe(
-      "Planning horizon must be a whole number of days.",
+      "Planning horizon must be a whole number of days, at least 1.",
     );
     expect(writes()).toEqual([]);
   });
diff --git a/admin/src/settings/SettingsScreen.tsx b/admin/src/settings/SettingsScreen.tsx
index fdc16f2..57c46d4 100644
--- a/admin/src/settings/SettingsScreen.tsx
+++ b/admin/src/settings/SettingsScreen.tsx
@@ -262,7 +262,12 @@ function preferencesDraft(settings: HouseholdSettings): PreferencesDraft {
 /** Only the fields that differ from `loaded`, or a validation message. */
 function buildChanges(draft: PreferencesDraft, loaded: HouseholdSettings): SettingsChanges | string {
   const horizon = draft.horizon_days.trim();
-  if (!/^\d+$/.test(horizon)) return "Planning horizon must be a whole number of days.";
+  const horizonDays = Number(horizon);
+  // A digit string too long for a safe integer would become Infinity, which
+  // JSON serializes as null, and the API reads null as "reset to default".
+  if (!/^\d+$/.test(horizon) || !Number.isSafeInteger(horizonDays) || horizonDays < 1) {
+    return "Planning horizon must be a whole number of days, at least 1.";
+  }
   const times: [TimeField, string][] = [
     ["day_rollover_time", "Day rollover"],
     ...NOTIFICATIONS.map((n): [TimeField, string] => [n.time, `${n.label} time`]),
@@ -273,7 +278,7 @@ function buildChanges(draft: PreferencesDraft, loaded: HouseholdSettings): Setti
 
   const next: HouseholdSettings = {
     ...draft,
-    horizon_days: Number(horizon),
+    horizon_days: horizonDays,
     notify_target: draft.notify_target.trim(),
   };
   const changes: SettingsChanges = {};