Can't create system message using API

Product
1

I tried to create a System message in a conversation using the following request:

POST https://{{api_url}}/v2/conversations/{{conversation_id}}/messages

{
  "message_parts": [
    {
      "text": {
        "content": "Message from system"
      }
    }
  ],
  "actor_type": "system"
}

But I get:

{
  "code": 404,
  "status": "USER_NOT_FOUND",
  "message": "User Not Found"
}

According to the API docs, actor_id isn’t required when sending a system message:

actor_id
Mandatory attribute in the request body parameter if the value of message.actor_type is user or agent.

If I add "actor_id": "{{agent_id}}" to json then there is no error but it appears as a normal message from Agent.