C＃ - webClient.DownloadDataAsync在webbrowser控件中抛出Ecxception

Question

我正在尝试使用以下代码行从服务器下载json字符串

private void webBrowser1_Navigating(object sender, WebBrowserNavigatingEventArgs e)
{
    e.Cancel = true;
    WebClient client = new WebClient();

    client.DownloadDataCompleted += new DownloadDataCompletedEventHandler(client_DownloadDataCompleted);

    client.DownloadDataAsync(new Uri("https://myApp:8070/api/account"));
}

void client_DownloadDataCompleted(object sender, DownloadDataCompletedEventArgs e)
{
    string filepath = textBox1.Text;
    File.WriteAllBytes(filepath, e.Result);
    MessageBox.Show("File downloaded");
}

但是得到如下的例外

System.Net.WebException: 'The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.'

同样获得 DownloadStringAsync 的相同例外，但 DownloadFileAsync 工作正常。

我有没有想念？