AdaptiveCard 400 Bad Request C# when posting to Teams webhook

With the below code i am getting StatusCode: 400, ReasonPhrase: 'Bad Request' but no further info:

response = {StatusCode: 400, ReasonPhrase: 'Bad Request', Version: 1.1, Content: System.Net.Http.HttpConnectionResponseContent, Headers:
{
  Cache-Control: no-cache
  Pragma: no-cache
  Server: Microsoft-IIS/10.0
  request-id: 5a296706-4079-47ed-aa57-e50eb3dd78fb...
}

I have already setup the webhook in Teams, appreciate pointer in the right direction.

var card = new AdaptiveCard(new AdaptiveSchemaVersion(1,0));
var columnSet = new AdaptiveColumnSet();
var column = new AdaptiveColumn();
var text = new AdaptiveTextBlock("one");

column.Items.Add(text);
columnSet.Columns.Add(column);
card.Body.Add(columnSet);

try
{

    var httpContent = new StringContent(card.ToJson(), Encoding.UTF8, AdaptiveCard.ContentType);

    var response = await httpClient
                            .PostAsync(settings.TeamsWebhook,httpContent)
                            .ConfigureAwait(false);

    if (response.StatusCode == HttpStatusCode.OK)
    {
        // Do something with response. Example get content:
        // var responseContent = await response.Content.ReadAsStringAsync ().ConfigureAwait (false);
    }
}

The produced JSON is as follows:

{
  "type": "AdaptiveCard",
  "version": "1.0",
  "title": "test",
  "body": [
    {
      "type": "ColumnSet",
      "columns": [
        {
          "type": "Column",
          "items": [
            {
              "type": "TextBlock",
              "text": "one"
            }
          ]
        }
      ]
    }
  ]
}

I have also tried by adding a container type (as i cannot find the attatchment element as below) and get the same errror.

var card = new AdaptiveCard(new AdaptiveSchemaVersion(1,2));
            var columnSet = new AdaptiveColumnSet();
            var column = new AdaptiveColumn();
            var text = new AdaptiveTextBlock("one");
            var container = new AdaptiveContainer();
            column.Items.Add(text);
            columnSet.Columns.Add(column);
            container.Items.Add(columnSet);
            card.Body.Add(container);

Gives the following json:

{
  "type": "AdaptiveCard",
  "version": "1.0",
  "body": [
    {
      "type": "Container",
      "items": [
        {
          "type": "ColumnSet",
          "columns": [
            {
              "type": "Column",
              "items": [
                {
                  "type": "TextBlock",
                  "text": "one"
                }
              ]
            }
          ]
        }
      ]
    }
  ]
}