Assign task to vendor via API

Hello,

Via the API, I would like to change the assignee of task (human-translation) from a group to a vendor order template. When I try to do so, I do get a 422 response (Unprocessable Entity) with the following error:

{
    "errorCode": "invalid",
    "message": "Input validation error occurred. No further details available.",
    "details": []
}

What is the reason? Do I need to perform an other step before assigning?

Regards,

Laurent