diff --git a/lib/web/fetch/webidl.js b/lib/web/fetch/webidl.js
index 96ec7767a1a..13cafae6f1b 100644
--- a/lib/web/fetch/webidl.js
+++ b/lib/web/fetch/webidl.js
@@ -250,6 +250,7 @@ webidl.sequenceConverter = function (converter) {
     /** @type {Generator} */
     const method = typeof Iterable === 'function' ? Iterable() : V?.[Symbol.iterator]?.()
     const seq = []
+    let index = 0
 
     // 3. If method is undefined, throw a TypeError.
     if (
@@ -270,7 +271,7 @@ webidl.sequenceConverter = function (converter) {
         break
       }
 
-      seq.push(converter(value, prefix, argument))
+      seq.push(converter(value, prefix, `${argument}[${index++}]`))
     }
 
     return seq
diff --git a/test/fetch/headers.js b/test/fetch/headers.js
index b5574ac03df..8536dea7175 100644
--- a/test/fetch/headers.js
+++ b/test/fetch/headers.js
@@ -27,7 +27,7 @@ test('Headers initialization', async (t) => {
       throws(() => new Headers(['undici', 'fetch', 'fetch']), TypeError)
       throws(
         () => new Headers([0, 1, 2]),
-        TypeError('Headers contructor: init (0) is not iterable.')
+        TypeError('Headers contructor: init[0] (0) is not iterable.')
       )
     })
 
@@ -42,7 +42,7 @@ test('Headers initialization', async (t) => {
       const init = ['undici', 'fetch', 'fetch', 'undici']
       throws(
         () => new Headers(init),
-        TypeError('Headers contructor: init ("undici") is not iterable.')
+        TypeError('Headers contructor: init[0] ("undici") is not iterable.')
       )
     })
   })
diff --git a/test/webidl/errors.js b/test/webidl/errors.js
index 20a73a765a3..c5406c35325 100644
--- a/test/webidl/errors.js
+++ b/test/webidl/errors.js
@@ -30,3 +30,19 @@ describe('dictionary converters', () => {
     )
   })
 })
+
+describe('sequence converters', () => {
+  test('retains index', () => {
+    const { port1 } = new MessageChannel()
+
+    assert.throws(
+      () => new MessageEvent('type', { ports: [{}] }),
+      new TypeError('MessageEvent constructor: Expected eventInitDict.ports[0] ("{}") to be an instance of MessagePort.')
+    )
+
+    assert.throws(
+      () => new MessageEvent('type', { ports: [port1, {}] }),
+      new TypeError('MessageEvent constructor: Expected eventInitDict.ports[1] ("{}") to be an instance of MessagePort.')
+    )
+  })
+})
diff --git a/test/websocket/messageevent.js b/test/websocket/messageevent.js
index ae8c2227336..36cc5971e6d 100644
--- a/test/websocket/messageevent.js
+++ b/test/websocket/messageevent.js
@@ -111,13 +111,13 @@ test('test/parallel/test-worker-message-port.js', () => {
   })
   assert.throws(() => new MessageEvent('message', { ports: [null] }), {
     constructor: TypeError,
-    message: 'MessageEvent constructor: Expected eventInitDict.ports ("null") to be an instance of MessagePort.'
+    message: 'MessageEvent constructor: Expected eventInitDict.ports[0] ("null") to be an instance of MessagePort.'
   })
   assert.throws(() =>
     new MessageEvent('message', { ports: [{}] })
   , {
     constructor: TypeError,
-    message: 'MessageEvent constructor: Expected eventInitDict.ports ("{}") to be an instance of MessagePort.'
+    message: 'MessageEvent constructor: Expected eventInitDict.ports[0] ("{}") to be an instance of MessagePort.'
   })
 
   assert(new MessageEvent('message') instanceof Event)