C# HtmlAlityPack返回子节点的空列表

我正试图从amazon上抓取一些东西，但无法通过登录。HtmlAgilityPack正在返回子节点的空列表。下面是表单的HTML外观（为可读性而编辑）

<form name="signIn" method="post" novalidate action="https://www.amazon.com/ap/signin/..." class="auth-validate-form auth-real-time-validation a-spacing-none">
    <input type="hidden" name="appActionToken" value="..." />
    <input type="hidden" name="appAction" value="SIGNIN" />
    <input type="hidden" name="openid.pape.max_auth_age" value="ape:MA==">
    <input type="hidden" name="openid.return_to" value="...">
    <input type="hidden" name="prevRID" value="...">

class Program
{
    static void Main(string[] args)
    {
        var amazonHomeUri = new Uri("https://www.amazon.com");
        var amazonHomeDoc = GetHtml(amazonHomeUri);
        var signInNode = amazonHomeDoc.DocumentNode.CssSelect("#nav-link-yourAccount").First();
        var signInUrl = signInNode.Attributes["href"].Value;
        var amazonSignInDoc = GetHtml(new Uri(amazonHomeUri, signInUrl));
        var loginForm = amazonSignInDoc.DocumentNode.CssSelect("form[name='signIn']").First();
        var postUrl = loginForm.Attributes["action"];

        //THIS IS WHERE THE ERROR HAPPENS
        //THERE ARE CERTAINLY INPUTS INSIDE THE FORM
        foreach(var input in loginForm.SelectNodes("input"))
        {
            System.Console.WriteLine(input.Attributes["name"]);
        }
    }
    private static HtmlAgilityPack.HtmlDocument GetHtml(Uri uri)
    {
        string html;
        WebRequest request = WebRequest.Create(uri);
        using (WebResponse response = request.GetResponse())
        {
            Stream dataStream = response.GetResponseStream();
            using (var reader = new StreamReader(dataStream))
            {
                html = reader.ReadToEnd();
            }
        }
        var doc = new HtmlAgilityPack.HtmlDocument();
        doc.LoadHtml(html);
        return doc;
    }
}