Cannot attach cookie header

Hi! I'm trying to send POST request to server and I need to attach Cookie header to my request if I receive it from server earlier. But that's trouble - I get an exception:

 ArgumentException: Cannot overwrite header: COOKIE
 UnityEngine.WWW.CheckSecurityOnHeaders

at WWW constructor's call:

 _www = new WWW(link, data, headers);

Can anybody help me to fix this? I have no idea...