ReplacePlayerForConnection is unspawned?

[magic lintel]

Hello there! I'm trying to replace the player object for the connection of a client, but the connection seems to be null?
I tried following this forum post (https://forum.unity.com/threads/connectiontoclient-is-null.483886/), and this GitHub issue (https://github.com/MirrorNetworking/Mirror/issues/3105)
Making the replace function a command seemed to get me a bit further, but still don't have the desired behaviour. Here's my code:

[Command(requiresAuthority = false)]
    public void CmdReplacePlayer(GameObject newController)
    {
        NetworkManagerSchabby.singleton.ReplacePlayer(GetComponent<NetworkIdentity>().connectionToClient, newController);
    }

public void ReplacePlayer(NetworkConnectionToClient conn, GameObject newPrefab)
    {
        Debug.Log(conn);
        Debug.Log(conn.identity);
        Debug.Log(conn.identity.gameObject);

        GameObject oldPlayer = conn.identity.gameObject;
        NetworkServer.ReplacePlayerForConnection(conn, Instantiate(newPrefab), true);
        Destroy(oldPlayer, 0.1f);
    }

The error:

When I run it on host, the game continues on, but there's no player object. Running on client kicks me from the server.

Maybe it has something to do with running it without authority?