Also got this error, when starting a game with prowess modifier:
```
ERR [string "code/ground.lua"]:1572: attempt to index a nil value
Stack traceback:
[string "code/game.lua"]:213: in function <[string "code/game.lua"]:197>
[string "code/game.lua"]:321: in function 'drw_level'
[string "code/game.lua"]:3097: in function 'drw_grid'
[string "code/ground.lua"]:1572: in function 'clear_dig'
```