calling 'base_attribute' on bad self (invalid entity)

[Ablu]

I spotted this error in the server logs:

[01:28:24] [WRN] Lua Script Error
     Script  : scripts/maps/sandbox.lua
     Error   : /home/mana/sourceoftales//scripts/attributes.lua:111: calling 'base_attribute' on bad self (invalid entity)
stack traceback:
        [C]: in function 'base_attribute'
        /home/mana/sourceoftales//scripts/attributes.lua:111: in function 'give_experience'
        /home/mana/sourceoftales//scripts/attributes.lua:166: in function </home/mana/sourceoftales//scripts/attributes.lua:126>
        (tail call): ?
        /home/mana/sourceoftales//scripts/abilities/strike.lua:68: in function </home/mana/sourceoftales//scripts/abilities/strike.lua:26>