Bot Builder v4：SendActivityAsync() 中的“BadRequest”

我有一个基于 Echo 机器人（csharp_dotnetcore 示例）的机器人，可以从 Teams 客户端运行和访问。我试图在“回声”之后发送一张简单的卡片（来自 Cards Bot），但我收到一个“BadRequest”错误......有什么问题？

我的代码：

protected override async Task OnMessageActivityAsync(ITurnContext<IMessageActivity> turnContext, CancellationToken cancellationToken)
{
    // OK
    await turnContext.SendActivityAsync(MessageFactory.Text($"Echo1: {turnContext.Activity.Text}"), cancellationToken);

    // 'BAD REQUEST'
    var attachments = new List<Attachment>();
    var message = MessageFactory.Attachment(attachments);
    message.Attachments.Add(GetAnimationCard().ToAttachment());
    await turnContext.SendActivityAsync(message, cancellationToken);
}

public static AnimationCard GetAnimationCard()
{
    var animationCard = new AnimationCard
    {
        Title = "Microsoft Bot Framework",
        Subtitle = "Animation Card",
        Image = new ThumbnailUrl
        {
            Url = "https://docs.microsoft.com/en-us/bot-framework/media/how-it-works/architecture-resize.png",
        },
        Media = new List<MediaUrl>
        {
            new MediaUrl()
            {
                Url = "http://i.giphy.com/Ki55RUbOV5njy.gif",
            },
        },
    };

    return animationCard;
}

谢谢，

迭戈