Letters not appearing when exporting

I’m attempting to create a typeface that is mainly built off of thin lines (see photo attached) but when I export it, it is all invisible except the parts that have filled with black (this is an error, I don’t want any colour to fill the shapes) any help is greatly appreciated as I have no idea what’s happening.

Fonts can’t contain open Paths. This might help: https://glyphsapp.com/learn/monoline