Istambul texture is TW_Crecent texture, and it haven't got any edge because it is multitextured -Multitextured maps haven't got edge-
Just make a 1x1 image of a
LIME GREEN dot with the name "istambul"
R:0 G:255 B:0
And then place it in Textures\Edges folder.
Or just place this image in Textures\Edges folder