调用FedEx运费报价API返回400状态码时反序列化对象失败的问题求助

我翻遍谷歌都没找到解决办法，只好来这儿求助了！

先贴一下我的核心代码：

try
{
    response = client.PostAsync("", content).Result;  // 阻塞调用！程序会在这里等待响应或超时
    if (response.IsSuccessStatusCode)
    {
        var contents = response.Content.ReadAsStringAsync().Result;
        FxRateReturn = JsonConvert.DeserializeObject<FedExFreightQuoteAPI_inbound.Root>(contents);
    }
    else
    {
        var contents = response.Content.ReadAsStringAsync().Result;
        FxRateReturn = JsonConvert.DeserializeObject<FedExFreightQuoteAPI_inbound.Root>(contents);
        throw new Exception("Error retrurned by the API: \n" + response.StatusCode.ToString() + response.ReasonPhrase.ToString() + FxRateReturn.errors.FirstOrDefault().message);
    }
    return FxRateReturn;
}
catch (Exception ex)
{
    throw new Exception(ex.Message, ex);
}

现在问题出在else分支里：当response.IsSuccessStatusCode为false时，我尝试读取响应内容并反序列化，但总是失败。报错信息是：

ERROR: Unexpected character encountered while parsing value: . Path '', line 0, position 0.

我用Postman测试过同样的请求，接口返回的错误响应明明是标准的JSON格式，但用我的代码调用时，响应内容就变成了一堆奇怪的字符，根本不是JSON。我实在搞不懂哪里出问题了！

对了，再贴一下我的HttpClient相关声明代码，方便大家排查：

HttpClient client = new HttpClient();
client.BaseAddress = new Uri(APIURL + "/rate/v1/rates/quotes");
// 添加JSON格式的Accept头
client.DefaultRequestHeaders.Accept.Add(
    new MediaTypeWithQualityHeaderValue("application/json"));
client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", p_Token);

var content = new StringContent(JsonConvert.SerializeObject(p_FedExOutbound), Encoding.UTF8, "application/json");
HttpResponseMessage response;

备注：内容来源于stack exchange，提问作者Be05x5