x11vnc Screen Black at Login

I have ssh access to an Ubuntu 16.04 box and I am normally able to coect, via x11vnc, using -display :0 -coect [server.tld]. It isn't a headless system and this only seems to work, when there is someone logged in. When the machine is at the login screen, however, the vnc screen is black, with a cursor. I have tried

    sudo x11vnc -auth /var/run/lightdm/root/:0 -display :0 -forever 
    -bg -coect [server.tld]