Fragment not supported in conversation

Is there a way to respond to an email message through the API? I’m currently trying to send the following JSON structure:

{
  "message_parts": [
    {
      "text": {
        "content": "$2"
      }
    }
  ],
  "message_type": "normal",
  "actor_type": "agent",
  "user_id": "b7bfa8b4-2a15-4dca-8e41-e46b4b8f03a7",
  "actor_id": "b7bfa8b4-2a15-4dca-8e41-e46b4b8f03a7"
}

However, I’m receiving the following error message:

{
  "code": 400,
  "status": "INVALID_VALUE",
  "message": "Fragment not supported in conversation"
}

Is there a way to address this issue?