Unable to set nav border to none

Under the Nav Options, when the slider for Nav Border Width is all the way to the left, the nav still has a border of 2 or 3 pixels. This only appears to be the case after a border width has been set, and I then want to change it back to 0px or “none”. On the live view, when adjusting the slider, it shows no border when moved all the way to the left, but when publishing the change, the border returns.

Any advice would be welcome.
Thanks
Jason