diff --git a/websocket-sharp/Net/HttpUtility.cs b/websocket-sharp/Net/HttpUtility.cs
index b83694ab..002aad7c 100644
--- a/websocket-sharp/Net/HttpUtility.cs
+++ b/websocket-sharp/Net/HttpUtility.cs
@@ -134,6 +134,27 @@ namespace WebSocketSharp.Net
                  : -1;
     }
 
+    private static string htmlAttributeEncode (string s)
+    {
+      var buff = new StringBuilder ();
+
+      foreach (var c in s) {
+        buff.Append (
+          c == '"'
+          ? """
+          : c == '&'
+            ? "&"
+            : c == '<'
+              ? "&lt;"
+              : c == '>'
+                ? "&gt;"
+                : c.ToString ()
+        );
+      }
+
+      return buff.ToString ();
+    }
+
     private static void initEntities ()
     {
       // Build the dictionary of HTML entity references.