diff --git a/lib/web/cookies/util.js b/lib/web/cookies/util.js
index 9ed048be874..055337a5e5a 100644
--- a/lib/web/cookies/util.js
+++ b/lib/web/cookies/util.js
@@ -324,19 +324,27 @@ function stringify (cookie) {
   }
 
   for (const part of cookie.unparsed) {
-    if (!part.includes('=')) {
-      throw new Error('Invalid unparsed')
-    }
-
-    const [key, ...value] = part.split('=')
+    if (part.includes('=')) {
+      const [key, ...value] = part.split('=')
 
-    const trimmedKey = key.trim()
-    const joinedValue = value.join('=')
+      const trimmedKey = key.trim()
+      if (trimmedKey.length === 0) {
+        throw new Error('Invalid unparsed')
+      }
+      const joinedValue = value.join('=')
 
-    validateCookieName(trimmedKey)
-    validateCookieValue(joinedValue)
+      validateCookieName(trimmedKey)
+      validateCookieValue(joinedValue)
 
-    out.push(`${trimmedKey}=${joinedValue}`)
+      out.push(`${trimmedKey}=${joinedValue}`)
+    } else {
+      const trimmedKey = part.trim()
+      if (trimmedKey.length === 0) {
+        throw new Error('Invalid unparsed')
+      }
+      validateCookieName(trimmedKey)
+      out.push(trimmedKey)
+    }
   }
 
   return out.join('; ')
diff --git a/test/cookie/cookies.js b/test/cookie/cookies.js
index 549627cf97a..841f4626aa2 100644
--- a/test/cookie/cookies.js
+++ b/test/cookie/cookies.js
@@ -782,3 +782,27 @@ test('Set-Cookie parser ignores an unparseable Expires', () => {
     value: 'a3fWa'
   }])
 })
+
+test('Cookie setCookie supports valueless unparsed attributes (e.g. Partitioned)', () => {
+  const headers = new Headers()
+  setCookie(headers, {
+    name: 'session',
+    value: 'abc',
+    secure: true,
+    path: '/',
+    unparsed: ['Partitioned']
+  })
+  assert.equal(headers.get('Set-Cookie'), 'session=abc; Secure; Path=/; Partitioned')
+
+  assert.throws(() => {
+    setCookie(headers, { name: 'session', value: 'abc', unparsed: [''] })
+  }, /Invalid unparsed/)
+
+  assert.throws(() => {
+    setCookie(headers, { name: 'session', value: 'abc', unparsed: ['  '] })
+  }, /Invalid unparsed/)
+
+  assert.throws(() => {
+    setCookie(headers, { name: 'session', value: 'abc', unparsed: ['=val'] })
+  }, /Invalid unparsed/)
+})