diff --git a/websocket-sharp/Net/HttpListenerResponse.cs b/websocket-sharp/Net/HttpListenerResponse.cs
index 0db7b7e6..a5427520 100644
--- a/websocket-sharp/Net/HttpListenerResponse.cs
+++ b/websocket-sharp/Net/HttpListenerResponse.cs
@@ -237,6 +237,9 @@ namespace WebSocketSharp.Net
     ///   The default value is <see langword="null"/>.
     ///   </para>
     /// </value>
+    /// <exception cref="InvalidOperationException">
+    /// The response is already being sent.
+    /// </exception>
     /// <exception cref="ObjectDisposedException">
     /// This instance is closed.
     /// </exception>