Straight transparent line appearing above Plank.

Tried doing this from another question:
"Just go to 'Setting > Window manager Tweaks > Compositor' and uncheck 'Show shadows under dock windows'." but 'show shadows' was already unchecked. It appears only on top of programs, on desktop it's unnoticeable. I'm running Manjaro with xfce 4.12. Installed plank using yaourt -S plank.

Thanks for any help.