diff --git a/websocket-sharp/Server/WebSocketServiceManager.cs b/websocket-sharp/Server/WebSocketServiceManager.cs
index f6c191e5..cf8d033f 100644
--- a/websocket-sharp/Server/WebSocketServiceManager.cs
+++ b/websocket-sharp/Server/WebSocketServiceManager.cs
@@ -348,6 +348,23 @@ namespace WebSocketSharp.Server
       return ret;
     }
 
+    private bool canSet (out string message)
+    {
+      message = null;
+
+      if (_state == ServerState.Start) {
+        message = "The server has already started.";
+        return false;
+      }
+
+      if (_state == ServerState.ShuttingDown) {
+        message = "The server is shutting down.";
+        return false;
+      }
+
+      return true;
+    }
+
     #endregion
 
     #region Internal Methods