unity 4.3 sprite script

i'm have successfully animated a sprite sheet but i can't use the script to control the animations, for example if want to press a button to make my character walk and call the animation which moves his legs. i tried to do something like this:

 Animator anim;
 anim = GetComponent<Animator>();
 and in the update function
 if(Input.GetButtonDown("walk"))
 {
 anim.SetTrigger("go");    
 }

where go is a .anim file in my assets that already has an animation set, but when i press the the button i get get a warning at run time "Parameter 'go' does not exist" although there is a go.anim file in my assets. so what can i be doing wrong?