Runtime error attack

ID:273983
 
Nov 1 2011, 9:13 am
Runtime Error: 
runtime error: Cannot read null.defense
proc name: Attack (/mob/proc/Attack)
  usr: King-okal (/mob)
  src: King-okal (/mob)
  call stack:
King-okal (/mob): Attack(null)
Attack (/obj/Skills/Attack): DblClick("Skills", "skills.info1", "icon-x=24;icon-y=18;left=1")

All Revelvant Code:
        statpanel("Skills")
        stat(src.skills)
mob/var/list/skills=list(new/obj/Skills/Attack)
obj/Skills
    Attack
        icon='AttackImg.gif'
        DblClick()
            usr.Attack()
mob
    proc
        Attack(mob/M in get_step(usr, usr.dir))
            var/damage=usr.strength - M.defense
            M.health -= damage
            M.DC()
            M<<output("[src] attacked you for [damage]!","dmgo")

well i was seeing my attack verb and i clicked it and the runtime error came,It may be because i wasnt infront of an npc but i dont want that happening when someone just clicks attack
Nov 1 2011, 9:26 am
Do a check to see if M exists.

if(!M) return
Nov 1 2011, 9:27 am
In response to Albro1
Works, Thanks!!
Nov 1 2011, 9:36 am
In response to King-okal
No problem.