diff --git a/lib/server.ts b/lib/server.ts
index 99b3f471..ba096f2e 100644
--- a/lib/server.ts
+++ b/lib/server.ts
@@ -160,7 +160,8 @@ function parseSessionId(data: string) {
 export abstract class BaseServer extends EventEmitter {
   public opts: ServerOptions;
 
-  protected clients: any;
+  // TODO for the next major release: use a Map instead
+  protected clients: Record<string, Socket>;
   public clientsCount: number;
   protected middlewares: Middleware[] = [];
 
@@ -590,7 +591,7 @@ export abstract class BaseServer extends EventEmitter {
       debug("upgrading existing transport");
 
       const transport = new WebTransport(session, stream, reader);
-      client.maybeUpgrade(transport);
+      client._maybeUpgrade(transport);
     }
   }
 
@@ -857,7 +858,7 @@ export class Server extends BaseServer {
 
         const transport = this.createTransport(req._query.transport, req);
         transport.perMessageDeflate = this.opts.perMessageDeflate;
-        client.maybeUpgrade(transport);
+        client._maybeUpgrade(transport);
       }
     } else {
       const closeConnection = (errorCode, errorContext) =>
diff --git a/lib/socket.ts b/lib/socket.ts
index acdd0a61..0febe90e 100644
--- a/lib/socket.ts
+++ b/lib/socket.ts
@@ -26,8 +26,8 @@ export class Socket extends EventEmitter {
   public transport: Transport;
 
   private server: Server;
-  private upgrading = false;
-  private upgraded = false;
+  /* private */ upgrading = false;
+  /* private */ upgraded = false;
   private writeBuffer: Packet[] = [];
   private packetsFn: SendCallback[] = [];
   private sentCallbackFn: SendCallback[][] = [];
@@ -264,7 +264,7 @@ export class Socket extends EventEmitter {
    * @param {Transport} transport
    * @api private
    */
-  private maybeUpgrade(transport) {
+  /* private */ _maybeUpgrade(transport: Transport) {
     debug(
       'might upgrade socket transport from "%s" to "%s"',
       this.transport.name,
diff --git a/lib/userver.ts b/lib/userver.ts
index 98380fbd..9d1eeb77 100644
--- a/lib/userver.ts
+++ b/lib/userver.ts
@@ -137,6 +137,7 @@ export class uServer extends BaseServer {
 
       if (req._query.sid) {
         debug("setting new request for existing client");
+        // @ts-ignore
         this.clients[req._query.sid].transport.onRequest(req);
       } else {
         const closeConnection = (errorCode, errorContext) =>
@@ -194,7 +195,7 @@ export class uServer extends BaseServer {
         } else {
           debug("upgrading existing transport");
           transport = this.createTransport(req._query.transport, req);
-          client.maybeUpgrade(transport);
+          client._maybeUpgrade(transport);
         }
       } else {
         transport = await this.handshake(