diff --git a/websocket-sharp/Server/WebSocketSessionManager.cs b/websocket-sharp/Server/WebSocketSessionManager.cs
index 812161c4..f4c9212a 100644
--- a/websocket-sharp/Server/WebSocketSessionManager.cs
+++ b/websocket-sharp/Server/WebSocketSessionManager.cs
@@ -364,6 +364,23 @@ namespace WebSocketSharp.Server
       return ret;
     }
 
+    private bool canSet (out string message)
+    {
+      message = null;
+
+      if (_state == ServerState.Start) {
+        message = "The service has already started.";
+        return false;
+      }
+
+      if (_state == ServerState.ShuttingDown) {
+        message = "The service is shutting down.";
+        return false;
+      }
+
+      return true;
+    }
+
     private static string createID ()
     {
       return Guid.NewGuid ().ToString ("N");