Hi i need some help with this code.

When i attack
Attack(mob/M)
M.lifeforce -= usr.strength - M.defence
M.Die()


it gets on to the die proc and it says can not read null.dfence thats for the monster and it happens with the attack when i use a more advanced attack proc.

mob/monster
Click()
usr.Attack()
saibaman
icon = 'saibaman.dmi'
icon_state = ""
strength = 4
lifeforce = 20
maxlifeforce = 20
defence = 4
zeni = 10

it doesnt read the variable do u know why?