diff --git a/websocket-sharp/Net/HttpListenerResponse.cs b/websocket-sharp/Net/HttpListenerResponse.cs
index 85d75c3f..f26f1234 100644
--- a/websocket-sharp/Net/HttpListenerResponse.cs
+++ b/websocket-sharp/Net/HttpListenerResponse.cs
@@ -554,7 +554,15 @@ namespace WebSocketSharp.Net
     ///   </para>
     /// </value>
     /// <exception cref="ArgumentException">
-    /// The value specified for a set operation is not an absolute URL.
+    ///   <para>
+    ///   The value specified for a set operation is an empty string.
+    ///   </para>
+    ///   <para>
+    ///   -or-
+    ///   </para>
+    ///   <para>
+    ///   The value specified for a set operation is not an absolute URL.
+    ///   </para>
     /// </exception>
     /// <exception cref="InvalidOperationException">
     /// The response is already being sent.