如何更改WebClient类的标头?

我试图更改WebClient的标头,但它总是引发异常

The Content-Type header cannot be changed from its default value for this request

我怎么能修好它?

//Webclient do the post to server.
WebClient wc = new WebClient();

//Header
wc.Headers["Content-Type"] = "application/x-www-form-urlencoded;charset=ISO-8859-1";