C# Grab element attribute by class name

So I want to grab the value of 'data-sitekey' from the snippet below.

<div class="g-recaptcha" data-sitekey="6LdC0iQUAAAAAOYmRv34KSLDe-7DmQrUSYJH8eB_"></div>

I've tried using a Web Browser as well as the HtmlAgilityPack but no luck. This is my code:

HtmlWeb web = new HtmlWeb();
                HtmlAgilityPack.HtmlDocument doc = web.Load(url);

                var headerNames = doc.DocumentNode.SelectNodes("//div[@class='g-recaptcha']").ToList();

                foreach (var item in headerNames)
                {
                    string hrefValue = item.GetAttributeValue("data-sitekey", string.Empty);
                    Console.WriteLine(hrefValue);
                }