diff --git a/packages/engine.io/lib/transports-uws/polling.ts b/packages/engine.io/lib/transports-uws/polling.ts
index ba49015c6..0e272f38b 100644
--- a/packages/engine.io/lib/transports-uws/polling.ts
+++ b/packages/engine.io/lib/transports-uws/polling.ts
@@ -53,9 +53,9 @@ export class Polling extends Transport {
     // remove the reference to the ServerResponse object (as the first request of the session is kept in memory by default)
     req.res = null;
 
-    if (req.getMethod() === "get") {
+    if (req.method === "GET") {
       this.onPollRequest(req, res);
-    } else if (req.getMethod() === "post") {
+    } else if (req.method === "POST") {
       this.onDataRequest(req, res);
     } else {
       res.writeStatus("500 Internal Server Error");
diff --git a/packages/engine.io/lib/userver.ts b/packages/engine.io/lib/userver.ts
index 76ecc9911..001755b51 100644
--- a/packages/engine.io/lib/userver.ts
+++ b/packages/engine.io/lib/userver.ts
@@ -181,6 +181,11 @@ export class uServer extends BaseServer {
 
     req.res = res;
 
+    // the uWS.HttpRequest object cannot be accessed once the handler has returned (e.g. with an asynchronous middleware or allowRequest() function)
+    const secWebSocketKey = req.getHeader("sec-websocket-key");
+    const secWebSocketProtocol = req.getHeader("sec-websocket-protocol");
+    const secWebSocketExtensions = req.getHeader("sec-websocket-extensions");
+
     const callback = async (errorCode, errorContext) => {
       if (errorCode !== undefined) {
         this.emit("connection_error", {
@@ -237,22 +242,24 @@ export class uServer extends BaseServer {
 
       this.emit("headers", additionalHeaders, req);
 
-      // calling writeStatus() triggers the flushing of any header added in a middleware
-      req.res.writeStatus("101 Switching Protocols");
+      res.cork(() => {
+        // calling writeStatus() triggers the flushing of any header added in a middleware
+        req.res.writeStatus("101 Switching Protocols");
 
-      Object.keys(additionalHeaders).forEach((key) => {
-        req.res.writeHeader(key, additionalHeaders[key]);
-      });
+        Object.keys(additionalHeaders).forEach((key) => {
+          req.res.writeHeader(key, additionalHeaders[key]);
+        });
 
-      res.upgrade(
-        {
-          transport,
-        },
-        req.getHeader("sec-websocket-key"),
-        req.getHeader("sec-websocket-protocol"),
-        req.getHeader("sec-websocket-extensions"),
-        context,
-      );
+        res.upgrade(
+          {
+            transport,
+          },
+          secWebSocketKey,
+          secWebSocketProtocol,
+          secWebSocketExtensions,
+          context,
+        );
+      });
     };
 
     this._applyMiddlewares(req, res, (err) => {
diff --git a/packages/engine.io/test/middlewares.js b/packages/engine.io/test/middlewares.js
index 60e1cc6b2..69f76fac7 100644
--- a/packages/engine.io/test/middlewares.js
+++ b/packages/engine.io/test/middlewares.js
@@ -56,6 +56,55 @@ describe("middlewares", () => {
     });
   });
 
+  it("should apply an asynchronous middleware (polling)", (done) => {
+    const engine = listen((port) => {
+      engine.use((req, res, next) => {
+        res.setHeader("foo", "bar");
+        setTimeout(next, 10);
+      });
+
+      request
+        .get(`http://localhost:${port}/engine.io/`)
+        .query({ EIO: 4, transport: "polling" })
+        .end((err, res) => {
+          expect(err).to.be(null);
+          expect(res.status).to.eql(200);
+          expect(res.headers["foo"]).to.eql("bar");
+
+          if (engine.httpServer) {
+            engine.httpServer.close();
+          }
+          done();
+        });
+    });
+  });
+
+  it("should apply an asynchronous middleware (websocket)", (done) => {
+    const engine = listen((port) => {
+      engine.use((req, res, next) => {
+        res.setHeader("foo", "bar");
+        setTimeout(next, 10);
+      });
+
+      const socket = new WebSocket(
+        `ws://localhost:${port}/engine.io/?EIO=4&transport=websocket`,
+      );
+
+      socket.on("upgrade", (res) => {
+        expect(res.headers["foo"]).to.eql("bar");
+
+        if (engine.httpServer) {
+          engine.httpServer.close();
+        }
+        done();
+      });
+
+      socket.on("open", () => {
+        socket.close();
+      });
+    });
+  });
+
   it("should apply all middlewares in order", (done) => {
     const engine = listen((port) => {
       let count = 0;