Can't unset Trello card cover color using api

Question

We use trello cover color to indicate roles.
If cover color changed without authority it should be changed back.
I tried to simulate this using postman and following request, but it is not working:

PUT https://api.trello.com/1/cards/<card id>?token=<token>&key=<key>
Accept: application/json
---------- Body ----------------

{"cover":{"color": null}}

this also not working:

{"cover":{"color": ""}}

yellow cover color in output:

{
    "cover": {
        "color": "yellow",
    },
}

I tried using different color from api, it works, but i can't UNSET color.

Answer 1

This works fine:

{"cover": ""}