Version mkgmap-r4701 was committed by gerd on Mon, 10 May 2021 fix possible empty rectangles : overSizePolyHole_v3.patch by Ticker Berkin "A large multipolygon (larger than the maximum size for a level 1 subdiv) exists, and is broken into some similar size pieces and some smaller pieces to expose inners/holes. The same problem can be caused by any polygons having these size properties." http://www.mkgmap.org.uk/websvn/revision.php?repname=mkgmap&rev=4701