Windows 8 HttpClient.PostAsync-为什么会引发ArgumentException

我有其他的表单帖子可以正常工作，但出于某种原因，这篇帖子会抛出一条“System.ArgumentException:值不在预期范围内”消息

有什么不对劲吗

using (HttpClient client = new HttpClient())
{
    HttpMultipartFormDataContent data = new HttpMultipartFormDataContent();
    data.Add(new HttpStringContent(about ?? String.Empty), "about");
    data.Add(new HttpStringContent(displayName ?? String.Empty), "name");
    data.Add(new HttpStringContent(email ?? String.Empty), "email");
    data.Add(new HttpStringContent(firstName ?? String.Empty), "firstName");
    data.Add(new HttpStringContent(lastName ?? String.Empty), "lastName");
    data.Add(new HttpStringContent(phone ?? String.Empty), "phone");
    data.Add(new HttpStringContent(isPublic.ToString()), "isPublic");
    data.Add(new HttpStringContent("true"), "isPrimaryProfile");

    Uri uri = new Uri(url, UriKind.Absolute);
    HttpResponseMessage responseMessage = await client.PostAsync(uri, data);

}

---

啊，显然空字符串不是HttpStringContent的有效值