diff --git a/packages/socket.io/lib/namespace.ts b/packages/socket.io/lib/namespace.ts
index d7ae1308fe..245551e61b 100644
--- a/packages/socket.io/lib/namespace.ts
+++ b/packages/socket.io/lib/namespace.ts
@@ -161,7 +161,7 @@ export class Namespace<
     SocketData
   >;
 
-  protected _fns: Array<
+  private _fns: Array<
     (
       socket: Socket<ListenEvents, EmitEvents, ServerSideEvents, SocketData>,
       next: (err?: ExtendedError) => void,
@@ -187,6 +187,23 @@ export class Namespace<
     this._initAdapter();
   }
 
+  protected static copyMiddleware<
+    ListenEvents extends EventsMap,
+    EmitEvents extends EventsMap,
+    ServerSideEvents extends EventsMap,
+    SocketData
+  >(
+    destination: Namespace<
+      ListenEvents,
+      EmitEvents,
+      ServerSideEvents,
+      SocketData
+    >,
+    source: Namespace<ListenEvents, EmitEvents, ServerSideEvents, SocketData>
+  ) {
+    source._fns.forEach((fn) => destination.use(fn));
+  }
+
   /**
    * Initializes the `Adapter` for this nsp.
    * Run upon changing adapter by `Server#adapter`
diff --git a/packages/socket.io/lib/parent-namespace.ts b/packages/socket.io/lib/parent-namespace.ts
index 6139b8e969..43ac6be350 100644
--- a/packages/socket.io/lib/parent-namespace.ts
+++ b/packages/socket.io/lib/parent-namespace.ts
@@ -67,7 +67,7 @@ export class ParentNamespace<
   ): Namespace<ListenEvents, EmitEvents, ServerSideEvents, SocketData> {
     debug("creating child namespace %s", name);
     const namespace = new Namespace(this.server, name);
-    this._fns.forEach((fn) => namespace.use(fn));
+    Namespace.copyMiddleware(namespace, this);
     this.listeners("connect").forEach((listener) =>
       namespace.on("connect", listener),
     );