diff --git a/websocket-sharp/Net/WebHeaderCollection.cs b/websocket-sharp/Net/WebHeaderCollection.cs
index bf681d1a..894a1cc1 100644
--- a/websocket-sharp/Net/WebHeaderCollection.cs
+++ b/websocket-sharp/Net/WebHeaderCollection.cs
@@ -811,7 +811,10 @@ namespace WebSocketSharp.Net
 
     private void checkRestricted (string name)
     {
-      if (!_internallyUsed && isRestricted (name, true))
+      if (_internallyUsed)
+        return;
+
+      if (isRestricted (name, true))
         throw new ArgumentException ("This header must be modified with the appropiate property.");
     }