diff --git a/websocket-sharp/Net/WebHeaderCollection.cs b/websocket-sharp/Net/WebHeaderCollection.cs
index bc4f6f48..2ea8f7be 100644
--- a/websocket-sharp/Net/WebHeaderCollection.cs
+++ b/websocket-sharp/Net/WebHeaderCollection.cs
@@ -782,6 +782,23 @@ namespace WebSocketSharp.Net
       base.Add (name, checkValue (value));
     }
 
+    private void checkAllowed (string name)
+    {
+      if (_state == HttpHeaderType.Unspecified)
+        return;
+
+      var headerType = getHeaderType (name);
+
+      if (headerType == HttpHeaderType.Unspecified)
+        return;
+
+      if (_state != headerType) {
+        var msg = "This collection does not allow the header.";
+
+        throw new InvalidOperationException (msg);
+      }
+    }
+
     private static string checkName (string name)
     {
       if (name == null)