diff --git a/EchoTcpServer/Program.cs b/EchoTcpServer/Program.cs
index 5966c579..036f1c4c 100644
--- a/EchoTcpServer/Program.cs
+++ b/EchoTcpServer/Program.cs
@@ -13,7 +13,7 @@ public class EchoServer
 {
     private readonly int _port;
     private TcpListener _listener;
-    private CancellationTokenSource _cancellationTokenSource;
+    private readonly CancellationTokenSource _cancellationTokenSource;
 
 
     public EchoServer(int port)
@@ -170,4 +170,4 @@ public void Dispose()
         StopSending();
         _udpClient.Dispose();
     }
-}
\ No newline at end of file
+}