diff --git a/websocket-sharp/Net/HttpListenerResponse.cs b/websocket-sharp/Net/HttpListenerResponse.cs
index a49e3ed0..01b989db 100644
--- a/websocket-sharp/Net/HttpListenerResponse.cs
+++ b/websocket-sharp/Net/HttpListenerResponse.cs
@@ -556,6 +556,9 @@ namespace WebSocketSharp.Net
     /// <exception cref="ArgumentException">
     /// The value specified for a set operation is not an absolute URL.
     /// </exception>
+    /// <exception cref="InvalidOperationException">
+    /// The response is already being sent.
+    /// </exception>
     /// <exception cref="ObjectDisposedException">
     /// This instance is closed.
     /// </exception>