Custom Header with WWW Get

I found a code example someone put up that said you can add a header to your Get request by leaving the postdata parameter null. What's happening to me is that when I include the headers I got a 404 error. But without the header I get a 401, which is expected, the header is for authorization. I've tried this with curl so I know the url and header value are correct.

var headers = new Hashtable(); headers.Add("Authorization", "Bearer MySessionID");

WWW www = new WWW("https://na10.salesforce.com/services/data/v20.0/sobjects/Account/myaccountid", null,headers);