diff --git a/src/iris/poll.ts b/src/iris/poll.ts
index 71ab6be..f96718a 100644
--- a/src/iris/poll.ts
+++ b/src/iris/poll.ts
@@ -15,11 +15,19 @@ export async function fetchFastTransferFeeBps(
   const url = `${irisBaseUrl(useSandbox)}burn/USDC/fees/${sourceDomain}/${destDomain}`;
   const res = await fetch(url);
   if (!res.ok) {
-    throw new Error(`Iris fees request failed: ${res.status} ${res.statusText}`);
+    throw new IrisRequestError(
+      `Iris fees request failed: ${res.status} ${res.statusText}`,
+      res.status,
+    );
   }
   const body = (await res.json()) as IrisFeesResponse;
   const fastEntry = body.find((entry) => entry.finalityThreshold <= 1000);
-  return fastEntry?.minimumFee ?? 0;
+  if (!fastEntry) {
+    throw new IrisRequestError(
+      `Iris fees response has no fast-transfer tier for domain pair ${sourceDomain} -> ${destDomain}`,
+    );
+  }
+  return fastEntry.minimumFee;
 }
 
 /**
diff --git a/tests/poll.test.ts b/tests/poll.test.ts
index e133283..7217c4d 100644
--- a/tests/poll.test.ts
+++ b/tests/poll.test.ts
@@ -56,25 +56,30 @@ describe("fetchFastTransferFeeBps", () => {
     expect(feeBps).toBe(2);
   });
 
-  it("returns 0 when no entry at or under the fast finality threshold exists", async () => {
+  it("rejects when no entry at or under the fast finality threshold exists", async () => {
     global.fetch = vi.fn().mockResolvedValue({
       ok: true,
       json: async () => [{ finalityThreshold: 2000, minimumFee: 5 }],
     }) as unknown as typeof fetch;
 
-    const feeBps = await fetchFastTransferFeeBps(26, 27, true);
+    const err = await fetchFastTransferFeeBps(26, 27, true).catch((e) => e);
 
-    expect(feeBps).toBe(0);
+    expect(err).toBeInstanceOf(IrisRequestError);
+    expect(err.message).toContain("26 -> 27");
   });
 
-  it("throws when the request fails", async () => {
+  it("throws IrisRequestError with the HTTP status when the request fails", async () => {
     global.fetch = vi.fn().mockResolvedValue({
       ok: false,
       status: 500,
       statusText: "Internal Server Error",
     }) as unknown as typeof fetch;
 
-    await expect(fetchFastTransferFeeBps(26, 27, true)).rejects.toThrow("Iris fees request failed");
+    const err = await fetchFastTransferFeeBps(26, 27, true).catch((e) => e);
+
+    expect(err).toBeInstanceOf(IrisRequestError);
+    expect(err.message).toContain("Iris fees request failed");
+    expect(err.status).toBe(500);
   });
 });