diff --git a/websocket-sharp/Net/WebHeaderCollection.cs b/websocket-sharp/Net/WebHeaderCollection.cs
index 06180931..7540d7af 100644
--- a/websocket-sharp/Net/WebHeaderCollection.cs
+++ b/websocket-sharp/Net/WebHeaderCollection.cs
@@ -1635,7 +1635,10 @@ namespace WebSocketSharp.Net
     /// </exception>
     public void Set (HttpResponseHeader header, string value)
     {
-      doWithCheckingState (setWithoutCheckingName, Convert (header), value, true, true);
+      var key = header.ToString ();
+      var name = getHeaderName (key);
+
+      doWithCheckingState (setWithoutCheckingName, name, value, true, true);
     }
 
     /// <summary>