diff --git a/websocket-sharp/Net/HttpUtility.cs b/websocket-sharp/Net/HttpUtility.cs
index 15ea0557..3859598b 100644
--- a/websocket-sharp/Net/HttpUtility.cs
+++ b/websocket-sharp/Net/HttpUtility.cs
@@ -1305,7 +1305,10 @@ namespace WebSocketSharp.Net
 
     public static string UrlPathEncode (string s)
     {
-      if (s == null || s.Length == 0)
+      if (s == null)
+        throw new ArgumentNullException ("s");
+
+      if (s.Length == 0)
         return s;
 
       var buff = new StringBuilder ();