为什么我的WebAPI服务器的HttpWebRequest POST方法失败了?

我已经成功地从我的WebAPI项目(“GET”)中接收到数据,但我发布的尝试不起作用。以下是相关的服务器/WebAPI代码:

public Department Add(Department item)
{
    if (item == null)
    {
        throw new ArgumentNullException("item");
    }
    departments.Add(item);
    return item; 
}

…当从客户端调用此代码时,在“departments.Add(item);”行失败:

const string uri = "http://localhost:48614/api/departments";
var dept = new Department();
dept.Id = 8;
dept.AccountId = "99";
dept.DeptName = "Something exceedingly funky";

var webRequest = (HttpWebRequest)WebRequest.Create(uri);
webRequest.Method = "POST";
var deptSerialized = JsonConvert.SerializeObject(dept); // <-- This is JSON.NET; it works (deptSerialized has the JSONized versiono of the Department object created above)
using (StreamWriter sw = new StreamWriter(webRequest.GetRequestStream()))
{
    sw.Write(deptSerialized);
}
HttpWebResponse httpWebResponse = webRequest.GetResponse() as HttpWebResponse;
using (StreamReader sr = new StreamReader(httpWebResponse.GetResponseStream()))
{
    if (httpWebResponse.StatusCode != HttpStatusCode.OK)
    {
        string message = String.Format("POST failed. Received HTTP {0}", httpWebResponse.StatusCode);
        throw new ApplicationException(message);
    }  
    MessageBox.Show(sr.ReadToEnd());
}

…在“HttpWebResponse HttpWebResponse=webRequest.GetResponse()as HttpWebResponse;”行失败。

服务器上的错误消息是departments为null;deptSerialized正在填充JSON“记录”,所以…这里缺少什么?

更新

指定ContentType确实解决了这个难题。此外,StatusCode是“Created”,这使得上面的代码抛出了一个异常,所以我将其更改为:

using (StreamReader sr = new StreamReader(httpWebResponse.GetResponseStream()))
{
    MessageBox.Show(String.Format("StatusCode == {0}", httpWebResponse.StatusCode));
    MessageBox.Show(sr.ReadToEnd());
}

…显示“StatusCode==Created”,后面是我创建的JSON“record”(数组成员?term.?)。