AWS LexV2 CDK/CloudFormation error when using Image Response Cards

I am deploying a Lex V2 bot with AWS CDK and want my bot to have buttons for eliciting slots, but for some reason I get an error:

DevBot Resource handler returned message: 

"Importing CDK-DevBot failed due to [There was an error importing the bot. 
Make sure that the imported bot and contents of the zip file are correct, then try your request again.]. 
The import could not be completed." 

(RequestToken: ebd3354f-6169-922a-d0f9-d14690671e25, HandlerErrorCode: InvalidRequest)

The relevant part of the CloudFormation template: "Message"

"MessageGroupsList: [{
  "Message": {
    "ImageResponseCard": {
      "Buttons": [
        {
          "Text": "1.0.3",
          "Value": "1.0.3"
        },
        {
          "Text": "1.0.5",
          "Value": "1.0.5"
        }
      ],
      "Title": "Title"
    },
    "PlainTextMessage": {
      "Value": "Please enter the issue number"
    }
  }
}]

If I remove "ImageResponseCard" then it deploys okay. Otherwise, I get the error.

Has anybody else had this problem and found a way to overcome it?