diff --git a/websocket-sharp/Net/HttpUtility.cs b/websocket-sharp/Net/HttpUtility.cs
index de4dcdf4..9b044436 100644
--- a/websocket-sharp/Net/HttpUtility.cs
+++ b/websocket-sharp/Net/HttpUtility.cs
@@ -907,6 +907,9 @@ namespace WebSocketSharp.Net
       if (output == null)
         throw new ArgumentNullException ("output");
 
+      if (s.Length == 0)
+        return;
+
       output.Write (HtmlDecode (s));
     }