how to change FoV

so when i try to change fov with script i get some null error, here's the script:

 var mainCam : Camera;
 var normalPos : Vector3;
 var aimPos : Vector3;
 var weapon : GameObject;
 
 function Start()
 {
     mainCam = Camera.main;
 }
 
 function Update()
 {
     if(Input.GetButton("Fire2"))
     {
         weapon.transform.localPosition = Vector3.Lerp(weapon.transform.localPosition, aimPos, Time.deltaTime * 8);
         mainCam.fieldOfView = Mathf.Lerp(mainCam.fieldOfView, 45, Time.deltaTime * 10);
     }
     else
     {
         weapon.transform.localPosition = Vector3.Lerp(weapon.transform.localPosition, normalPos, Time.deltaTime * 8);
         mainCam.fieldOfView = Mathf.Lerp(mainCam.fieldOfView, 60, Time.deltaTime * 10);
     }
 }