HttpWebRequest.Header problem

I used HttpWebRequest.Header in my project , when I build to Windows Phone8 project , error occurred.
type System.Collections.Specialized.NameValueCollection doesn’t exist in target framework.
I check in .Net for Windows Phone API , the type of HttpWebRequest.Header is WebHeaderCollection. And the base class of WebHeaderCollection is Object . Why this error ocurred?

It’s a known bug. This happens because in .Net for desktop, WebHeaderCollection inherits from NameValueCollection: