diff --git a/cli/tests/unit/websocket_test.ts b/cli/tests/unit/websocket_test.ts
index 7bc53495af..a31e71e047 100644
--- a/cli/tests/unit/websocket_test.ts
+++ b/cli/tests/unit/websocket_test.ts
@@ -365,3 +365,42 @@ Deno.test(async function websocketTlsSocketWorks() {
 
   await finished;
 });
+
+// https://github.com/denoland/deno/issues/15340
+Deno.test(
+  async function websocketServerFieldInit() {
+    const ac = new AbortController();
+    const listeningDeferred = Promise.withResolvers<void>();
+
+    const server = Deno.serve({
+      handler: (req) => {
+        const { socket, response } = Deno.upgradeWebSocket(req, {
+          idleTimeout: 0,
+        });
+        socket.onopen = function () {
+          assert(typeof socket.url == "string");
+          assert(socket.readyState == WebSocket.OPEN);
+          assert(socket.protocol == "");
+          socket.close();
+        };
+        socket.onclose = () => ac.abort();
+        return response;
+      },
+      signal: ac.signal,
+      onListen: () => listeningDeferred.resolve(),
+      hostname: "localhost",
+      port: servePort,
+    });
+
+    await listeningDeferred.promise;
+    const deferred = Promise.withResolvers<void>();
+    const ws = new WebSocket(serveUrl);
+    assertEquals(ws.url, serveUrl);
+    ws.onerror = () => fail();
+    ws.onclose = () => {
+      deferred.resolve();
+    };
+
+    await Promise.all([deferred.promise, server.finished]);
+  },
+);
diff --git a/ext/http/01_http.js b/ext/http/01_http.js
index acaea5e7c0..309e44ee46 100644
--- a/ext/http/01_http.js
+++ b/ext/http/01_http.js
@@ -3,7 +3,6 @@
 import { core, internals, primordials } from "ext:core/mod.js";
 const { BadResourcePrototype, InterruptedPrototype, ops } = core;
 const { op_http_write } = Deno.core.ensureFastOps();
-import * as webidl from "ext:deno_webidl/00_webidl.js";
 import { InnerBody } from "ext:deno_fetch/22_body.js";
 import { Event, setEventTargetData } from "ext:deno_web/02_event.js";
 import { BlobPrototype } from "ext:deno_web/09_file.js";
@@ -29,6 +28,7 @@ import {
   _role,
   _server,
   _serverHandleIdleTimeout,
+  createWebSocketBranded,
   SERVER,
   WebSocket,
 } from "ext:deno_websocket/01_websocket.js";
@@ -472,7 +472,7 @@ function upgradeWebSocket(request, options = {}) {
     }
   }
 
-  const socket = webidl.createBranded(WebSocket);
+  const socket = createWebSocketBranded(WebSocket);
   setEventTargetData(socket);
   socket[_server] = true;
   socket[_idleTimeoutDuration] = options.idleTimeout ?? 120;
diff --git a/ext/websocket/01_websocket.js b/ext/websocket/01_websocket.js
index 477f93b93d..2aea080716 100644
--- a/ext/websocket/01_websocket.js
+++ b/ext/websocket/01_websocket.js
@@ -582,6 +582,20 @@ defineEventHandler(WebSocket.prototype, "open");
 webidl.configureInterface(WebSocket);
 const WebSocketPrototype = WebSocket.prototype;
 
+function createWebSocketBranded() {
+  const socket = webidl.createBranded(WebSocket);
+  socket[_rid] = undefined;
+  socket[_role] = undefined;
+  socket[_readyState] = CONNECTING;
+  socket[_extensions] = "";
+  socket[_protocol] = "";
+  socket[_url] = "";
+  socket[_binaryType] = "blob";
+  socket[_idleTimeoutDuration] = 0;
+  socket[_idleTimeoutTimeout] = undefined;
+  return socket;
+}
+
 export {
   _eventLoop,
   _idleTimeoutDuration,
@@ -592,6 +606,7 @@ export {
   _role,
   _server,
   _serverHandleIdleTimeout,
+  createWebSocketBranded,
   SERVER,
   WebSocket,
 };