diff --git a/src/error.test.js b/src/error.test.js
new file mode 100644
index 0000000..b69c36a
--- /dev/null
+++ b/src/error.test.js
@@ -0,0 +1,40 @@
+const assert = require('node:assert/strict')
+const { describe, it } = require('node:test')
+const TruepicWebhookVerifierError = require('./error')
+const verifyTruepicWebhook = require('./main')
+
+describe('TruepicWebhookVerifierError', () => {
+  const message = 'Signature is not valid'
+
+  it('sets the `message`', () => {
+    assert.strictEqual(
+      new TruepicWebhookVerifierError(message).message,
+      message,
+    )
+  })
+
+  it('sets the `name` to the class name', () => {
+    assert.strictEqual(
+      new TruepicWebhookVerifierError(message).name,
+      'TruepicWebhookVerifierError',
+    )
+  })
+
+  it('is an instance of `Error`', () => {
+    assert.ok(new TruepicWebhookVerifierError(message) instanceof Error)
+  })
+
+  it('captures a stack trace pointing at the caller', () => {
+    const error = new TruepicWebhookVerifierError(message)
+
+    assert.ok(error.stack.startsWith(`TruepicWebhookVerifierError: ${message}`))
+    assert.match(error.stack, /error\.test\.js/)
+  })
+
+  it('is exported from the main module', () => {
+    assert.strictEqual(
+      verifyTruepicWebhook.TruepicWebhookVerifierError,
+      TruepicWebhookVerifierError,
+    )
+  })
+})
diff --git a/src/main.js b/src/main.js
index 144c0db..67510a1 100644
--- a/src/main.js
+++ b/src/main.js
@@ -17,7 +17,7 @@ const TruepicWebhookVerifierError = require('./error')
  * @returns {Object} The parsed `timestamp` and `signature` values.
  */
 function parseHeader(header) {
-  if (!header?.length) {
+  if (typeof header !== 'string' || !header.length) {
     throw new TruepicWebhookVerifierError('Header is missing or empty')
   }
 
@@ -108,6 +108,13 @@ function verifyTimestamp({ timestamp, leewayMinutes }) {
  * @returns {true} If verification succeeds.
  */
 function verifySignature({ url, secret, body, timestamp, signature }) {
+  // Guard the secret before handing it to `createHmac`, which would otherwise
+  // throw a raw `TypeError` and break the documented contract that every
+  // failure is a `TruepicWebhookVerifierError`.
+  if (typeof secret !== 'string' || !secret.length) {
+    throw new TruepicWebhookVerifierError('Secret is missing or empty')
+  }
+
   // Rebuild the signature (SHA-256 HMAC digest) with a secret that only Truepic
   // and the intended receiver are privy to.
   const comparisonSignature = createHmac('sha256', secret)
diff --git a/src/main.test.js b/src/main.test.js
index 3095c15..4fc6984 100644
--- a/src/main.test.js
+++ b/src/main.test.js
@@ -67,9 +67,60 @@ describe('verifyTruepicWebhook', () => {
         true,
       )
     })
+
+    it('defaults to a 5 minute leeway if none is given', (t) => {
+      t.mock.timers.enable({
+        apis: ['Date'],
+        now: sentAtMs + 1000 * 60 * 5,
+      })
+
+      assert.strictEqual(
+        verifyTruepicWebhook({
+          url,
+          secret,
+          header,
+          body,
+        }),
+        true,
+      )
+    })
+
+    it('accepts a webhook that arrives instantly with no leeway', (t) => {
+      t.mock.timers.enable({ apis: ['Date'], now: sentAtMs })
+
+      assert.strictEqual(
+        verifyTruepicWebhook({
+          url,
+          secret,
+          header,
+          body,
+          leewayMinutes: 0,
+        }),
+        true,
+      )
+    })
   })
 
   describe('throws a `TruepicWebhookVerifierError`', () => {
+    it('that is an instance of the exported error class', () => {
+      assert.throws(
+        () =>
+          verifyTruepicWebhook({
+            url,
+            secret,
+            header: '',
+            body,
+            leewayMinutes,
+          }),
+        (error) => {
+          assert.ok(error instanceof TruepicWebhookVerifierError)
+          assert.ok(error instanceof Error)
+
+          return true
+        },
+      )
+    })
+
     it('if the `header` is missing', () => {
       assert.throws(
         () =>
@@ -98,6 +149,20 @@ describe('verifyTruepicWebhook', () => {
       )
     })
 
+    it('if the `header` is not a string', () => {
+      assert.throws(
+        () =>
+          verifyTruepicWebhook({
+            url,
+            secret,
+            header: ['t=1698259719,s=abc'],
+            body,
+            leewayMinutes,
+          }),
+        new TruepicWebhookVerifierError('Header is missing or empty'),
+      )
+    })
+
     it('if the `header` cannot be parsed into timestamp and signature', () => {
       assert.throws(
         () =>
@@ -130,6 +195,38 @@ describe('verifyTruepicWebhook', () => {
       )
     })
 
+    it('if the `header` is missing the timestamp part entirely', () => {
+      assert.throws(
+        () =>
+          verifyTruepicWebhook({
+            url,
+            secret,
+            header: ',s=test',
+            body,
+            leewayMinutes,
+          }),
+        new TruepicWebhookVerifierError(
+          'Header cannot be parsed into timestamp and signature',
+        ),
+      )
+    })
+
+    it('if the `header` is missing the signature part entirely', () => {
+      assert.throws(
+        () =>
+          verifyTruepicWebhook({
+            url,
+            secret,
+            header: 't=1698259719,',
+            body,
+            leewayMinutes,
+          }),
+        new TruepicWebhookVerifierError(
+          'Header cannot be parsed into timestamp and signature',
+        ),
+      )
+    })
+
     it('if the `header` is missing the timestamp (`t`)', () => {
       assert.throws(
         () =>
@@ -158,6 +255,20 @@ describe('verifyTruepicWebhook', () => {
       )
     })
 
+    it('if the `header` timestamp part has no `=`', () => {
+      assert.throws(
+        () =>
+          verifyTruepicWebhook({
+            url,
+            secret,
+            header: 't,s=test',
+            body,
+            leewayMinutes,
+          }),
+        new TruepicWebhookVerifierError('Timestamp is missing or empty'),
+      )
+    })
+
     it('if the `header` timestamp (`t`) is not a number', () => {
       assert.throws(
         () =>
@@ -200,6 +311,20 @@ describe('verifyTruepicWebhook', () => {
       )
     })
 
+    it('if the `header` signature part has no `=`', () => {
+      assert.throws(
+        () =>
+          verifyTruepicWebhook({
+            url,
+            secret,
+            header: 't=1698259719,s',
+            body,
+            leewayMinutes,
+          }),
+        new TruepicWebhookVerifierError('Signature is missing or empty'),
+      )
+    })
+
     it('if the webhook arrives more than 5 minutes late', (t) => {
       t.mock.timers.enable({
         apis: ['Date'],
@@ -242,6 +367,44 @@ describe('verifyTruepicWebhook', () => {
       )
     })
 
+    it('if the webhook arrives more than 5 minutes late and no leeway is given', (t) => {
+      t.mock.timers.enable({
+        apis: ['Date'],
+        now: sentAtMs + 1000 * 60 * 5 + 1,
+      })
+
+      assert.throws(
+        () =>
+          verifyTruepicWebhook({
+            url,
+            secret,
+            header,
+            body,
+          }),
+        new TruepicWebhookVerifierError(
+          'Timestamp is not within allowed window',
+        ),
+      )
+    })
+
+    it('if the webhook is late at all and no leeway is allowed', (t) => {
+      t.mock.timers.enable({ apis: ['Date'], now: sentAtMs + 1 })
+
+      assert.throws(
+        () =>
+          verifyTruepicWebhook({
+            url,
+            secret,
+            header,
+            body,
+            leewayMinutes: 0,
+          }),
+        new TruepicWebhookVerifierError(
+          'Timestamp is not within allowed window',
+        ),
+      )
+    })
+
     it('if the `url` is not where the request was sent', () => {
       assert.throws(
         () =>
@@ -298,6 +461,48 @@ describe('verifyTruepicWebhook', () => {
       )
     })
 
+    it('if the `secret` is missing', () => {
+      assert.throws(
+        () =>
+          verifyTruepicWebhook({
+            url,
+            secret: undefined,
+            header,
+            body,
+            leewayMinutes,
+          }),
+        new TruepicWebhookVerifierError('Secret is missing or empty'),
+      )
+    })
+
+    it('if the `secret` is empty', () => {
+      assert.throws(
+        () =>
+          verifyTruepicWebhook({
+            url,
+            secret: '',
+            header,
+            body,
+            leewayMinutes,
+          }),
+        new TruepicWebhookVerifierError('Secret is missing or empty'),
+      )
+    })
+
+    it('if the `secret` is not a string', () => {
+      assert.throws(
+        () =>
+          verifyTruepicWebhook({
+            url,
+            secret: 1698259719,
+            header,
+            body,
+            leewayMinutes,
+          }),
+        new TruepicWebhookVerifierError('Secret is missing or empty'),
+      )
+    })
+
     it('if the `secret` is not what was used to sign', () => {
       assert.throws(
         () =>