diff --git a/lib/uws.ts b/lib/uws.ts
index 1abc6a0184..6e400c7f77 100644
--- a/lib/uws.ts
+++ b/lib/uws.ts
@@ -25,7 +25,9 @@ export function patchAdapter(app /* : TemplatedApp */) {
     if (isNew) {
       socket.conn.on("upgrade", () => {
         const rooms = this.sids.get(id);
-        subscribe(this.nsp.name, socket, isNew, rooms);
+        if (rooms) {
+          subscribe(this.nsp.name, socket, isNew, rooms);
+        }
       });
     }
   };
diff --git a/test/uws.ts b/test/uws.ts
index de185d7c03..814a2467fd 100644
--- a/test/uws.ts
+++ b/test/uws.ts
@@ -186,6 +186,12 @@ describe("socket.io with uWebSocket.js-based engine", () => {
     io.to("room1").emit("hello");
   });
 
+  it("should not crash when socket is disconnected before the upgrade", (done) => {
+    client.on("disconnect", () => done());
+
+    io.of("/").sockets.get(client.id)!.disconnect();
+  });
+
   it("should serve static files", (done) => {
     const clientVersion = require("socket.io-client/package.json").version;