diff --git a/lib/fetch/headers.js b/lib/fetch/headers.js
index 43860c5d98a..41ae9b02368 100644
--- a/lib/fetch/headers.js
+++ b/lib/fetch/headers.js
@@ -528,8 +528,10 @@ Object.defineProperties(Headers.prototype, {
 
 webidl.converters.HeadersInit = function (V) {
   if (webidl.util.Type(V) === 'Object') {
-    if (V[Symbol.iterator]) {
-      return webidl.converters['sequence<sequence<ByteString>>'](V)
+    const iterator = Reflect.get(V, Symbol.iterator)
+
+    if (typeof iterator === 'function') {
+      return webidl.converters['sequence<sequence<ByteString>>'](V, iterator.bind(V))
     }
 
     return webidl.converters['record<ByteString, ByteString>'](V)
diff --git a/lib/fetch/webidl.js b/lib/fetch/webidl.js
index a93a25505fc..0978e67f26f 100644
--- a/lib/fetch/webidl.js
+++ b/lib/fetch/webidl.js
@@ -218,7 +218,7 @@ webidl.util.IntegerPart = function (n) {
 
 // https://webidl.spec.whatwg.org/#es-sequence
 webidl.sequenceConverter = function (converter) {
-  return (V) => {
+  return (V, Iterable) => {
     // 1. If Type(V) is not Object, throw a TypeError.
     if (webidl.util.Type(V) !== 'Object') {
       throw webidl.errors.exception({
@@ -229,7 +229,7 @@ webidl.sequenceConverter = function (converter) {
 
     // 2. Let method be ? GetMethod(V, @@iterator).
     /** @type {Generator} */
-    const method = V?.[Symbol.iterator]?.()
+    const method = typeof Iterable === 'function' ? Iterable() : V?.[Symbol.iterator]?.()
     const seq = []
 
     // 3. If method is undefined, throw a TypeError.
@@ -273,8 +273,8 @@ webidl.recordConverter = function (keyConverter, valueConverter) {
     const result = {}
 
     if (!types.isProxy(O)) {
-      // Object.keys only returns enumerable properties
-      const keys = Object.keys(O)
+      // 1. Let desc be ? O.[[GetOwnProperty]](key).
+      const keys = [...Object.getOwnPropertyNames(O), ...Object.getOwnPropertySymbols(O)]
 
       for (const key of keys) {
         // 1. Let typedKey be key converted to an IDL value of type K.
diff --git a/test/fetch/headers.js b/test/fetch/headers.js
index fcdf4b7a820..d3e27fa348d 100644
--- a/test/fetch/headers.js
+++ b/test/fetch/headers.js
@@ -719,3 +719,30 @@ test('When the value is updated, update the cache', (t) => {
   headers.append('d', 'd')
   deepStrictEqual([...headers], [...expected, ['d', 'd']])
 })
+
+test('Symbol.iterator is only accessed once', (t) => {
+  const { ok } = tspl(t, { plan: 1 })
+
+  const dict = new Proxy({}, {
+    get () {
+      ok(true)
+
+      return function * () {}
+    }
+  })
+
+  new Headers(dict) // eslint-disable-line no-new
+})
+
+test('Invalid Symbol.iterators', (t) => {
+  const { throws } = tspl(t, { plan: 3 })
+
+  throws(() => new Headers({ [Symbol.iterator]: null }), TypeError)
+  throws(() => new Headers({ [Symbol.iterator]: undefined }), TypeError)
+  throws(() => {
+    const obj = { [Symbol.iterator]: null }
+    Object.defineProperty(obj, Symbol.iterator, { enumerable: false })
+
+    new Headers(obj) // eslint-disable-line no-new
+  }, TypeError)
+})
diff --git a/types/webidl.d.ts b/types/webidl.d.ts
index 40cfe064f8f..f29bebbb1e8 100644
--- a/types/webidl.d.ts
+++ b/types/webidl.d.ts
@@ -5,7 +5,7 @@
  */
 type Converter<T> = (object: unknown) => T
 
-type SequenceConverter<T> = (object: unknown) => T[]
+type SequenceConverter<T> = (object: unknown, iterable?: IterableIterator<T>) => T[]
 
 type RecordConverter<K extends string, V> = (object: unknown) => Record<K, V>