Sorry I have no idea why that's happening. I'll keep looking at it but I don't know what's going on. I initialised all the variables in both 'obj_maptile_tile' and in the script 'map_sprites()' so I don't know.
According to online resources the DoAdd error is either not initialising variables before using them, or it's trying to add incompatible data types e.g. combine string ("words") with a number.
Afaik my code isn't doing either, so idk.