Oh my god. We're all idiots.
That "lake" in Montenegro that all of our old maps had isn't a lake at all. It's Montenegro itself. When the Ottomans owned the Balkans "Montenegro" was still their vassal, though at that time the only land the Prince/Duke/Count/whatever of Montenegro owned was that one "lake" looking province in the middle of what should be modern day Montenegro. None of us ever realized that it was just a map artifact from having to copy over maps and editing them for different time periods.