diff --git a/websocket-sharp/Server/WebSocketBehavior.cs b/websocket-sharp/Server/WebSocketBehavior.cs
index 0a5040d3..1f7b6b20 100644
--- a/websocket-sharp/Server/WebSocketBehavior.cs
+++ b/websocket-sharp/Server/WebSocketBehavior.cs
@@ -941,7 +941,7 @@ namespace WebSocketSharp.Server
     protected void CloseAsync (ushort code, string reason)
     {
       if (_websocket == null) {
-        var msg = "The session has not started yet.";
+        var msg = "The CloseAsync method is not available.";
 
         throw new InvalidOperationException (msg);
       }