diff --git a/README.md b/README.md
index 80d1b3f..8fd244d 100644
--- a/README.md
+++ b/README.md
@@ -87,7 +87,8 @@ This module works with `yarn` in PnP (plug'n play) mode too!
 ### Emit events
 
 You can emit events on the ThreadStream from your worker using [`worker.parentPort.postMessage()`](https://nodejs.org/api/worker_threads.html#workerparentport).
-The message (JSON object) must have the following data structure:
+Messages that do not carry a thread-stream protocol `code` are ignored.
+For custom events, the message (JSON object) must have the following data structure:
 
 ```js
 parentPort.postMessage({
diff --git a/index.js b/index.js
index db2b23b..d30d11c 100644
--- a/index.js
+++ b/index.js
@@ -174,6 +174,12 @@ function onWorkerMessage (msg) {
     return
   }
 
+  // Node.js watch mode may send internal worker messages that do not
+  // participate in thread-stream's worker protocol.
+  if (msg?.code == null) {
+    return
+  }
+
   switch (msg.code) {
     case 'READY':
       // Replace the FakeWeakRef with a
diff --git a/test/message-without-code.js b/test/message-without-code.js
new file mode 100644
index 0000000..d06db9d
--- /dev/null
+++ b/test/message-without-code.js
@@ -0,0 +1,19 @@
+'use strict'
+
+const { Writable } = require('stream')
+const { parentPort } = require('worker_threads')
+
+async function run () {
+  parentPort.postMessage({
+    internal: 'watch-mode'
+  })
+
+  return new Writable({
+    autoDestroy: true,
+    write (chunk, enc, cb) {
+      cb()
+    }
+  })
+}
+
+module.exports = run
diff --git a/test/watch-mode.test.js b/test/watch-mode.test.js
new file mode 100644
index 0000000..9c06fb4
--- /dev/null
+++ b/test/watch-mode.test.js
@@ -0,0 +1,28 @@
+'use strict'
+
+const { test } = require('tap')
+const { join } = require('path')
+const ThreadStream = require('..')
+
+test('ignores worker messages without a protocol code', function (t) {
+  t.plan(2)
+
+  const stream = new ThreadStream({
+    filename: join(__dirname, 'message-without-code.js'),
+    sync: false
+  })
+
+  const errors = []
+  stream.on('error', err => {
+    errors.push(err)
+  })
+
+  stream.on('ready', () => {
+    t.ok(stream.write('hello world\n'))
+    stream.end()
+  })
+
+  stream.on('finish', () => {
+    t.same(errors, [])
+  })
+})