botframework-sdk: Can't send custom Slack Message with version 3 of Bot Connector

I have a bot that replies with a message consisting only of attachments. When it works on Slack, it uses Slack attachment formatting quite heavily, thus I have to use ChannelData property.

In version 1 of BotConnector, the code was like this

    var reply = message.CreateReplyMessage();
    reply.Attachments = new List<Attachment>();
    var attachments = new List<object>(); //Slack-formatted attachments
    //filling attachments...
    reply.ChannelData = new {attachments};

and it worked. Now, in version 3 code has changed to

    var reply = activity.CreateReply();
    reply.Attachments = new List<Attachment>();
    var attachments = new List<object>(); //Slack-formatted attachments
    //filling attachments...
    reply.ChannelData = new {attachments};

    var connector = new ConnectorClient(new Uri(activity.ServiceUrl));
    await connector.Conversations.ReplyToActivityAsync(reply);

which, basically, boils down to using different method to create reply, and another one to send the reply back.

Now, the problem is, that I don’t get the reply back to Slack. Diagnostics in AppInsight show me that somewhere in Connector something like this happens:

Exception type: System.ArgumentNullException
Failed method: SlackChannel.SlackMapper+d__5.MoveNext
Exception Message: value cannot be null. Parameter name: source
ChannelData: {}
message: Invalid ChannelData

Please note that ChannelData in this diagnostics seems to be empty. So what I gather from all this is that something has changed in the way BotConnector processes ChannelData. How can I find out what exactly am I doing wrong?