diff --git a/websocket-sharp/WebSocket.cs b/websocket-sharp/WebSocket.cs
index d71b01a4..eb086310 100644
--- a/websocket-sharp/WebSocket.cs
+++ b/websocket-sharp/WebSocket.cs
@@ -445,7 +445,7 @@ namespace WebSocketSharp
         string msg = null;
 
         if (!_client) {
-          msg = "The set operation cannot be used by servers.";
+          msg = "This instance is not a client.";
           throw new InvalidOperationException (msg);
         }