webapi post请求返回Null异常

Question

我在Web API控制器中有一个简单的post方法，它接受HttpRequestMessage。不知何故，XML没有被解析为HttpRequestMessage。我已将应用配置为使用XmlSerializer

尝试将简单的XML传递给它但给我Null引用异常

public class SyncSubscriptionController : ApiController
{       
    [HttpPost]
    public HttpResponseMessage Post(HttpRequestMessage req)
    {    
        var data = req.Content.ReadAsStringAsync().Result;
    }
}

从Fiddler发送帖子请求：

User-Agent: Fiddler
Host: localhost:11033
Content-Length: 1852
Content-Type: text/xml

Body:
<customer>
<customer_id>1234</customer_id>
</customer>

Global.asax中

GlobalConfiguration.Configuration.Formatters.XmlFormatter.UseXmlSerializer = true;