diff --git a/src/lib/tail-stream.test.ts b/src/lib/tail-stream.test.ts
index 0f1424f..4638351 100644
--- a/src/lib/tail-stream.test.ts
+++ b/src/lib/tail-stream.test.ts
@@ -106,7 +106,7 @@ describe("streamTail", () => {
     // before we close the stream.
     await gen.next();
     driver.close();
-    await gen.return(undefined);
+    await gen.return?.(undefined);
     expect(fetchMock).toHaveBeenCalledTimes(1);
     const [calledUrl] = fetchMock.mock.calls[0] as [string];
     expect(calledUrl).toBe("/api/acks/stream");
@@ -121,7 +121,7 @@ describe("streamTail", () => {
     const gen = streamTail("ta1_acks");
     await gen.next();
     driver.close();
-    await gen.return(undefined);
+    await gen.return?.(undefined);
     expect(fetchMock).toHaveBeenCalledTimes(1);
     const [calledUrl] = fetchMock.mock.calls[0] as [string];
     expect(calledUrl).toBe("/api/ta1-acks/stream");
diff --git a/src/pages/Acks.test.tsx b/src/pages/Acks.test.tsx
index 4ca7eef..a7c726e 100644
--- a/src/pages/Acks.test.tsx
+++ b/src/pages/Acks.test.tsx
@@ -9,15 +9,19 @@ import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import { Acks } from "./Acks";
 import { api } from "@/lib/api";
 
-vi.mock("@/lib/api", async (importOriginal) => {
-  const actual = await importOriginal();
+vi.mock("@/lib/api", () => {
+  class ApiError extends Error {
+    constructor(public status: number, message: string) {
+      super(message);
+    }
+  }
   return {
-    ...actual,
     api: {
       isConfigured: true,
       listAcks: vi.fn(),
       getAck: vi.fn(),
     },
+    ApiError,
   };
 });
 
diff --git a/src/store/tail-store.ts b/src/store/tail-store.ts
index d6b2630..f2bda85 100644
--- a/src/store/tail-store.ts
+++ b/src/store/tail-store.ts
@@ -79,11 +79,11 @@ interface TailStore {
  */
 type RemitListItem = Remittance;
 
-function evictOldest<T>(
-  order: Array<string | number>,
-  dict: Record<string, T>,
+function evictOldest<K extends string | number, V>(
+  order: K[],
+  dict: Record<string, V>,
   batch: number,
-): { order: Array<string | number>; dict: Record<string, T> } {
+): { order: K[]; dict: Record<string, V> } {
   if (order.length <= TAIL_CAP) return { order, dict };
   // Drain down to the cap; never evict more than `batch` per call so a
   // 5-item overflow evicts 5, but a 1 000-item overflow evicts 100 in
@@ -93,7 +93,7 @@ function evictOldest<T>(
   const nextOrder = order.slice(toDrop);
   // Object.assign is consistently faster than `{ ...dict }` in V8 for
   // large dicts; we follow up with `delete` for each evicted id.
-  const nextDict: Record<string, T> = Object.assign({}, dict);
+  const nextDict: Record<string, V> = Object.assign({}, dict);
   for (const id of dropped) delete nextDict[String(id)];
   return { order: nextOrder, dict: nextDict };
 }
@@ -172,7 +172,7 @@ export const useTailStore = create<TailStore>((set) => ({
       const nextOrder = s.ackOrder.concat(a.id);
       if (nextOrder.length > TAIL_CAP) {
         const { order, dict } = evictOldest(nextOrder, nextAcks, EVICT_BATCH);
-        return { acks: dict, ackOrder: order as number[] };
+        return { acks: dict, ackOrder: order };
       }
       return { acks: nextAcks, ackOrder: nextOrder };
     }),
@@ -188,7 +188,7 @@ export const useTailStore = create<TailStore>((set) => ({
       const nextOrder = s.ta1AckOrder.concat(a.id);
       if (nextOrder.length > TAIL_CAP) {
         const { order, dict } = evictOldest(nextOrder, nextTa1Acks, EVICT_BATCH);
-        return { ta1Acks: dict, ta1AckOrder: order as number[] };
+        return { ta1Acks: dict, ta1AckOrder: order };
       }
       return { ta1Acks: nextTa1Acks, ta1AckOrder: nextOrder };
     }),