diff --git a/src/Expires.ts b/src/Expires.ts
index b64a30be..34de8c19 100644
--- a/src/Expires.ts
+++ b/src/Expires.ts
@@ -1,3 +1,28 @@
+import { InvalidChallengeError, PaymentExpiredError } from './Errors.js'
+
+/**
+ * Asserts that `expires` is present, well-formed, and not in the past.
+ *
+ * Throws `InvalidChallengeError` when missing or malformed,
+ * and `PaymentExpiredError` when the timestamp is in the past.
+ */
+export function assert(
+  expires: string | undefined,
+  challengeId?: string,
+): asserts expires is string {
+  if (!expires)
+    throw new InvalidChallengeError({
+      ...(challengeId && { id: challengeId }),
+      reason: 'missing required expires field',
+    })
+  if (Number.isNaN(new Date(expires).getTime()))
+    throw new InvalidChallengeError({
+      ...(challengeId && { id: challengeId }),
+      reason: 'malformed expires timestamp',
+    })
+  if (new Date(expires) < new Date()) throw new PaymentExpiredError({ expires })
+}
+
 /** Returns an ISO 8601 datetime string `n` days from now. */
 export function days(n: number) {
   return new Date(Date.now() + n * 24 * 60 * 60 * 1000).toISOString()
diff --git a/src/server/Mppx.test.ts b/src/server/Mppx.test.ts
index 68e0feaa..eab12cd2 100644
--- a/src/server/Mppx.test.ts
+++ b/src/server/Mppx.test.ts
@@ -422,6 +422,92 @@ describe('request handler', () => {
     `)
     expect((body as { detail: string }).detail).toContain('Payment expired at')
   })
+  test('returns 402 when credential challenge has no expires (fail-closed)', async () => {
+    const handle = Mppx.create({ methods: [method], realm, secretKey }).charge({
+      amount: '1000',
+      currency: asset,
+      expires: new Date(Date.now() + 60_000).toISOString(),
+      recipient: accounts[0].address,
+    })
+
+    // Get a valid challenge from the server to capture the exact request shape
+    const firstResult = await handle(new Request('https://example.com/resource'))
+    expect(firstResult.status).toBe(402)
+    if (firstResult.status !== 402) throw new Error()
+
+    const serverChallenge = Challenge.fromResponse(firstResult.challenge)
+
+    // Re-create the same challenge WITHOUT expires, with a valid HMAC
+    const { expires: _, ...rest } = serverChallenge
+    const challengeNoExpires = Challenge.from({
+      secretKey,
+      realm: rest.realm,
+      method: rest.method,
+      intent: rest.intent,
+      request: rest.request,
+      ...(rest.opaque && { meta: rest.opaque }),
+    })
+
+    const credential = Credential.from({
+      challenge: challengeNoExpires,
+      payload: { signature: '0x123', type: 'transaction' },
+    })
+
+    const result = await handle(
+      new Request('https://example.com/resource', {
+        headers: { Authorization: Credential.serialize(credential) },
+      }),
+    )
+
+    expect(result.status).toBe(402)
+    if (result.status !== 402) throw new Error()
+
+    const body = (await result.challenge.json()) as { title: string; detail: string }
+    expect(body.title).toBe('Invalid Challenge')
+    expect(body.detail).toContain('missing required expires')
+  })
+  test('returns 402 when credential challenge has malformed expires', async () => {
+    const handle = Mppx.create({ methods: [method], realm, secretKey }).charge({
+      amount: '1000',
+      currency: asset,
+      expires: new Date(Date.now() + 60_000).toISOString(),
+      recipient: accounts[0].address,
+    })
+
+    // Get a valid challenge from the server to capture the exact request shape
+    const firstResult = await handle(new Request('https://example.com/resource'))
+    expect(firstResult.status).toBe(402)
+    if (firstResult.status !== 402) throw new Error()
+
+    const serverChallenge = Challenge.fromResponse(firstResult.challenge)
+
+    // Re-create the challenge with a valid HMAC but inject a malformed expires
+    // by patching the challenge object after construction (bypasses zod at build time).
+    const challengeMalformed = {
+      ...serverChallenge,
+      expires: 'not-a-timestamp',
+    }
+
+    const credential = Credential.from({
+      challenge: challengeMalformed as any,
+      payload: { signature: '0x123', type: 'transaction' },
+    })
+
+    // Credential.serialize does not re-validate, so the malformed expires
+    // reaches the server. Deserialization rejects it via zod schema.
+    const result = await handle(
+      new Request('https://example.com/resource', {
+        headers: { Authorization: Credential.serialize(credential) },
+      }),
+    )
+
+    expect(result.status).toBe(402)
+    if (result.status !== 402) throw new Error()
+
+    const body = (await result.challenge.json()) as { title: string; detail: string }
+    expect(body.title).toBe('Malformed Credential')
+  })
+
   test('returns 402 when payload schema validation fails', async () => {
     const handle = Mppx.create({ methods: [method], realm, secretKey }).charge({
       amount: '1000',
diff --git a/src/server/Mppx.ts b/src/server/Mppx.ts
index 6df90eb5..59c5b4d2 100644
--- a/src/server/Mppx.ts
+++ b/src/server/Mppx.ts
@@ -418,14 +418,14 @@ function createMethodFn(parameters: createMethodFn.Parameters): createMethodFn.R
           }
         }
 
-        // Reject expired credentials
-        if (credential.challenge.expires && new Date(credential.challenge.expires) < new Date()) {
+        // Reject credentials without expires (fail-closed) or with expired timestamp
+        try {
+          Expires.assert(credential.challenge.expires, credential.challenge.id)
+        } catch (error) {
           const response = await transport.respondChallenge({
             challenge,
             input,
-            error: new Errors.PaymentExpiredError({
-              expires: credential.challenge.expires,
-            }),
+            error: error as Errors.PaymentError,
           })
           return { challenge: response, status: 402 }
         }
diff --git a/src/stripe/server/Charge.ts b/src/stripe/server/Charge.ts
index eac11803..096f05da 100644
--- a/src/stripe/server/Charge.ts
+++ b/src/stripe/server/Charge.ts
@@ -1,9 +1,6 @@
 import type * as Credential from '../../Credential.js'
-import {
-  PaymentActionRequiredError,
-  PaymentExpiredError,
-  VerificationFailedError,
-} from '../../Errors.js'
+import { PaymentActionRequiredError, VerificationFailedError } from '../../Errors.js'
+import * as Expires from '../../Expires.js'
 import type { LooseOmit, OneOf } from '../../internal/types.js'
 import * as Method from '../../Method.js'
 import type { StripeClient } from '../internal/types.js'
@@ -66,8 +63,7 @@ export function charge<const parameters extends charge.Parameters>(parameters: p
       const { challenge } = credential
       const { request } = challenge
 
-      if (challenge.expires && new Date(challenge.expires) < new Date())
-        throw new PaymentExpiredError({ expires: challenge.expires })
+      Expires.assert(challenge.expires, challenge.id)
 
       const parsed = Methods.charge.schema.credential.payload.safeParse(credential.payload)
       if (!parsed.success) throw new Error('Invalid credential payload: missing or malformed spt')
diff --git a/src/tempo/server/Charge.ts b/src/tempo/server/Charge.ts
index e99c8d45..bfdb88b0 100644
--- a/src/tempo/server/Charge.ts
+++ b/src/tempo/server/Charge.ts
@@ -9,7 +9,7 @@ import {
 import { tempo as tempo_chain } from 'viem/chains'
 import { Abis, Transaction } from 'viem/tempo'
 
-import { PaymentExpiredError } from '../../Errors.js'
+import * as Expires from '../../Expires.js'
 import type { LooseOmit, NoExtraKeys } from '../../internal/types.js'
 import * as Method from '../../Method.js'
 import * as Store from '../../Store.js'
@@ -118,7 +118,7 @@ export function charge<const parameters extends charge.Parameters>(
       const currency = challengeRequest.currency as `0x${string}`
       const recipient = challengeRequest.recipient as `0x${string}`
 
-      if (expires && new Date(expires) < new Date()) throw new PaymentExpiredError({ expires })
+      Expires.assert(expires, challenge.id)
 
       const memo = methodDetails?.memo as `0x${string}` | undefined