diff --git a/websocket-sharp/Net/HttpUtility.cs b/websocket-sharp/Net/HttpUtility.cs
index 97d73dc2..7d1b4001 100644
--- a/websocket-sharp/Net/HttpUtility.cs
+++ b/websocket-sharp/Net/HttpUtility.cs
@@ -846,6 +846,9 @@ namespace WebSocketSharp.Net
 
     public static void HtmlAttributeEncode (string s, TextWriter output)
     {
+      if (s == null)
+        throw new ArgumentNullException ("s");
+
       if (output == null)
         throw new ArgumentNullException ("output");