"Feed is read only" error with Google Maps API

I am beginning to use the google data API (specifically for the finance app). I can read my portfolio's just fine, so I am authenticating correctly (or so I think). However, when I try and create a portfolio, I get a 'feed is read-only' error. The constructor for the service:

public class FinanceService : Service, IService
{
    public FinanceService(string applicationName)
        : base ("finance", applicationName)
    {
        this.RequestFactory = new GDataGAuthRequestFactory("finance", applicationName) { ProtocolMajor = 3 };
    }
}

and saving it is

private const string _schema = "http://schemas.google.com/finance/2007";
private const string _feed = "http://finance.google.com/finance/feeds/default/portfolios";

AtomFeed atomFeed = new AtomFeed(new Uri(_feed), th开发者_StackOverflow社区is.FinanceService);

return this.FinanceService.Insert(atomFeed, this as AtomEntry) as PortfolioEntry;

Any idea why the atomFeed would come back ReadOnly? The credentials are legit, and I can get my current portfolios without a problem.