Borders do not accept custom countries

[Venom34]

I need help getting borders to work. I have a set of custom countries to put in, but if I put one of them in and then press "ok", the box becomes blank and the country doesn't get applied. Base game countries work just fine. I checked each country def file and I couldn't spot any issues.

[SeZu]

Countries are assigned only on road nodes. Designate the border at the extreme nodes of the roads, where the border of the country is supposed to be. Then do "Map -> Recompute map". On all other nodes, the country will be assigned automatically.