diff --git a/packages/engine.io/lib/userver.ts b/packages/engine.io/lib/userver.ts
index 6cef789bc..bbbb04d8a 100644
--- a/packages/engine.io/lib/userver.ts
+++ b/packages/engine.io/lib/userver.ts
@@ -185,13 +185,13 @@ export class uServer extends BaseServer {
         const client = this.clients[id];
         if (!client) {
           debug("upgrade attempt for closed client");
-          res.close();
+          return res.close();
         } else if (client.upgrading) {
           debug("transport has already been trying to upgrade");
-          res.close();
+          return res.close();
         } else if (client.upgraded) {
           debug("transport had already been upgraded");
-          res.close();
+          return res.close();
         } else {
           debug("upgrading existing transport");
           transport = this.createTransport(req._query.transport, req);
diff --git a/packages/engine.io/test/server.js b/packages/engine.io/test/server.js
index 898e53f67..1795b00bb 100644
--- a/packages/engine.io/test/server.js
+++ b/packages/engine.io/test/server.js
@@ -205,6 +205,26 @@ describe("server", () => {
       });
     });
 
+    it("should prevent the client from upgrading twice", (done) => {
+      engine = listen((port) => {
+        const client = new ClientSocket(`ws://localhost:${port}`);
+
+        client.on("upgrade", () => {
+          const socket = new WebSocket(
+            `ws://localhost:${port}/engine.io/?EIO=4&transport=websocket&sid=${client.id}`,
+          );
+
+          socket.on("error", () => {});
+
+          socket.on("close", () => {
+            client.close();
+
+            done();
+          });
+        });
+      });
+    });
+
     it("should disallow `__proto__` as transport (polling)", (done) => {
       const partialDone = createPartialDone(done, 2);