diff --git a/websocket-sharp/Net/HttpUtility.cs b/websocket-sharp/Net/HttpUtility.cs
index a3dfb9cf..17e6c923 100644
--- a/websocket-sharp/Net/HttpUtility.cs
+++ b/websocket-sharp/Net/HttpUtility.cs
@@ -857,7 +857,7 @@ namespace WebSocketSharp.Net
       if (output == null)
         throw new ArgumentNullException ("output");
 
-      output.Write (HtmlAttributeEncode (s));
+      output.Write (s.Length > 0 ? htmlAttributeEncode (s) : s);
     }
 
     /// <summary>