diff --git a/websocket-sharp/Net/WebHeaderCollection.cs b/websocket-sharp/Net/WebHeaderCollection.cs
index 13d51516..56ecd4e3 100644
--- a/websocket-sharp/Net/WebHeaderCollection.cs
+++ b/websocket-sharp/Net/WebHeaderCollection.cs
@@ -735,7 +735,10 @@ namespace WebSocketSharp.Net
     /// </exception>
     public string this[HttpResponseHeader header] {
       get {
-        return Get (Convert (header));
+        var key = header.ToString ();
+        var name = getHeaderName (key);
+
+        return Get (name);
       }
 
       set {