diff --git a/lib/web/fetch/index.js b/lib/web/fetch/index.js
index 5fbef495ad6..784e0c2cdbb 100644
--- a/lib/web/fetch/index.js
+++ b/lib/web/fetch/index.js
@@ -444,8 +444,7 @@ function fetching ({
   // 9. If request’s origin is "client", then set request’s origin to request’s
   // client’s origin.
   if (request.origin === 'client') {
-    // TODO: What if request.client is null?
-    request.origin = request.client?.origin
+    request.origin = request.client.origin
   }
 
   // 10. If all of the following conditions are true:
diff --git a/lib/web/fetch/util.js b/lib/web/fetch/util.js
index 299f4e64c81..e533ac957ca 100644
--- a/lib/web/fetch/util.js
+++ b/lib/web/fetch/util.js
@@ -255,16 +255,23 @@ function appendFetchMetadata (httpRequest) {
 
 // https://fetch.spec.whatwg.org/#append-a-request-origin-header
 function appendRequestOriginHeader (request) {
-  // 1. Let serializedOrigin be the result of byte-serializing a request origin with request.
+  // 1. Let serializedOrigin be the result of byte-serializing a request origin
+  //    with request.
+  // TODO: implement "byte-serializing a request origin"
   let serializedOrigin = request.origin
 
-  // 2. If request’s response tainting is "cors" or request’s mode is "websocket", then append (`Origin`, serializedOrigin) to request’s header list.
-  if (request.responseTainting === 'cors' || request.mode === 'websocket') {
-    if (serializedOrigin) {
-      request.headersList.append('origin', serializedOrigin, true)
-    }
+  // "'client' is changed to an origin during fetching."
+  // This doesn't happen in undici (in most cases) because undici, by default,
+  // has no concept of origin.
+  if (serializedOrigin === 'client') {
+    return
+  }
 
+  // 2. If request’s response tainting is "cors" or request’s mode is "websocket",
+  //    then append (`Origin`, serializedOrigin) to request’s header list.
   // 3. Otherwise, if request’s method is neither `GET` nor `HEAD`, then:
+  if (request.responseTainting === 'cors' || request.mode === 'websocket') {
+    request.headersList.append('origin', serializedOrigin, true)
   } else if (request.method !== 'GET' && request.method !== 'HEAD') {
     // 1. Switch on request’s referrer policy:
     switch (request.referrerPolicy) {
@@ -275,13 +282,16 @@ function appendRequestOriginHeader (request) {
       case 'no-referrer-when-downgrade':
       case 'strict-origin':
       case 'strict-origin-when-cross-origin':
-        // If request’s origin is a tuple origin, its scheme is "https", and request’s current URL’s scheme is not "https", then set serializedOrigin to `null`.
+        // If request’s origin is a tuple origin, its scheme is "https", and
+        // request’s current URL’s scheme is not "https", then set
+        // serializedOrigin to `null`.
         if (request.origin && urlHasHttpsScheme(request.origin) && !urlHasHttpsScheme(requestCurrentURL(request))) {
           serializedOrigin = null
         }
         break
       case 'same-origin':
-        // If request’s origin is not same origin with request’s current URL’s origin, then set serializedOrigin to `null`.
+        // If request’s origin is not same origin with request’s current URL’s
+        // origin, then set serializedOrigin to `null`.
         if (!sameOrigin(request, requestCurrentURL(request))) {
           serializedOrigin = null
         }
@@ -290,10 +300,8 @@ function appendRequestOriginHeader (request) {
         // Do nothing.
     }
 
-    if (serializedOrigin) {
-      // 2. Append (`Origin`, serializedOrigin) to request’s header list.
-      request.headersList.append('origin', serializedOrigin, true)
-    }
+    // 2. Append (`Origin`, serializedOrigin) to request’s header list.
+    request.headersList.append('origin', serializedOrigin, true)
   }
 }
 
diff --git a/lib/web/websocket/connection.js b/lib/web/websocket/connection.js
index a434e45ac82..5058fc58095 100644
--- a/lib/web/websocket/connection.js
+++ b/lib/web/websocket/connection.js
@@ -35,7 +35,7 @@ try {
  * @param {(response: any) => void} onEstablish
  * @param {Partial<import('../../types/websocket').WebSocketInit>} options
  */
-function establishWebSocketConnection (url, protocols, ws, onEstablish, options) {
+function establishWebSocketConnection (url, protocols, client, ws, onEstablish, options) {
   // 1. Let requestURL be a copy of url, with its scheme set to "http", if url’s
   //    scheme is "ws", and to "https" otherwise.
   const requestURL = url
@@ -48,6 +48,7 @@ function establishWebSocketConnection (url, protocols, ws, onEstablish, options)
   //    and redirect mode is "error".
   const request = makeRequest({
     urlList: [requestURL],
+    client,
     serviceWorkers: 'none',
     referrer: 'no-referrer',
     mode: 'websocket',
diff --git a/lib/web/websocket/websocket.js b/lib/web/websocket/websocket.js
index 892ce78c2db..97e86c3801e 100644
--- a/lib/web/websocket/websocket.js
+++ b/lib/web/websocket/websocket.js
@@ -124,6 +124,7 @@ class WebSocket extends EventTarget {
     this[kWebSocketURL] = new URL(urlRecord.href)
 
     // 11. Let client be this's relevant settings object.
+    const client = environmentSettingsObject.settingsObject
 
     // 12. Run this step in parallel:
 
@@ -132,6 +133,7 @@ class WebSocket extends EventTarget {
     this[kController] = establishWebSocketConnection(
       urlRecord,
       protocols,
+      client,
       this,
       (response) => this.#onConnectionEstablished(response),
       options
diff --git a/test/fetch/issue-rsshub-15532.js b/test/fetch/issue-rsshub-15532.js
new file mode 100644
index 00000000000..11491337de5
--- /dev/null
+++ b/test/fetch/issue-rsshub-15532.js
@@ -0,0 +1,25 @@
+'use strict'
+
+const { once } = require('node:events')
+const { createServer } = require('node:http')
+const { test } = require('node:test')
+const { fetch } = require('../..')
+const { tspl } = require('@matteo.collina/tspl')
+
+// https://github.com/DIYgod/RSSHub/issues/15532
+test('An invalid Origin header is not set', async (t) => {
+  const { deepStrictEqual } = tspl(t, { plan: 1 })
+
+  const server = createServer((req, res) => {
+    deepStrictEqual(req.headers.origin, undefined)
+
+    res.end()
+  }).listen(0)
+
+  await once(server, 'listening')
+  t.after(server.close.bind(server))
+
+  await fetch(`http://localhost:${server.address().port}`, {
+    method: 'POST'
+  })
+})