c# - UnassignedReferenceException on an assigned variable (unity 3D) -

i'm new unity (using 4.5) , i'm trying learn unity 3d. have small "game" want swing axe kill spiders. have created animation named "axeswing" , small script trigger animation when click left button. probleme have error saying variable "attackanimationclip" has nothing assigned it. can see on screen shot assigned , when left click animation trigger , come idleaxe animation.

here script:

[requirecomponent(typeof(animation))] public class attackanimation : monobehaviour {     public animationclip attackanimationclip;     // update called once per frame     void update () {         if (input.getmousebutton(0))             animation.play(attackanimationclip.name);         else if (isattackfinished)             animation.crossfade(animation.clip.name);      }      private bool isattackfinished     {         { return animation[attackanimationclip.name].time > animation[attackanimationclip.name].length;  }     } } 

since have animation attached gameobject can this:

 animation.play("axeswing");