[mkgmap-dev] Tile Border Artifact with --generate-sea

I have just noticed 'thin blue line' (I estimate about 1 pixel wide) along every tile edge )or border). This is more obvious when I 'zoom out' and it highlights my tiles because there is this blue sea-coloured border around them. I am using the Geofabrik GB OSM download and my relevant compile line is: --generate-sea:no-mp, extend-sea-sectors, close-gaps=10000. Currently using mkgmap version 1827 Has this been seen before and Is there a setting for me to avoid this?.