diff --git a/websocket-sharp/Net/HttpUtility.cs b/websocket-sharp/Net/HttpUtility.cs
index af2be570..b83694ab 100644
--- a/websocket-sharp/Net/HttpUtility.cs
+++ b/websocket-sharp/Net/HttpUtility.cs
@@ -1042,6 +1042,9 @@ namespace WebSocketSharp.Net
 
     public static void HtmlEncode (string s, TextWriter output)
     {
+      if (s == null)
+        throw new ArgumentNullException ("s");
+
       if (output == null)
         throw new ArgumentNullException ("output");