Camera switching from first person to 3rd, script not working, need help plz

function Update () { //Switch Cameras

    if(Input.GetButton ("Tab")) {

        GetComponent("Main Camera").enable = false;
        GetComponent("Camera2").enable = true;

    }
    else
    {
        GetComponent("Main Camera").enable = true;
        GetComponent("Camera2").enable = false;
    }

}

This is a function to switch between two cameras but it does not work, any ideas why? i get returned...

"NullReferenceException: Object reference not set to an instance of an object Boo.Lang.Runtime.RuntimeServices.Dispatch (System.Object target, System.String cacheKeyName, System.Type[] cacheKeyTypes, System.Object[] args, Boo.Lang.Runtime.DispatcherFactory factory) [0x00000] Boo.Lang.Runtime.RuntimeServices.Dispatch (System.Object target, System.String cacheKeyName, System.Object[] args, Boo.Lang.Runtime.DispatcherFactory factory) [0x00000] Boo.Lang.Runtime.RuntimeServices.SetProperty (System.Object target, System.String name, System.Object value) CameraSwitch.Update () (at Assets/_Scripts/PlayerScripts/CameraSwitch.js:10)"

Try this:

var FirstPerson : Camera;
var ThirdPerson : Camera;
var Active : boolean = false;

function Start(){
   FirstPerson.enabled = true;
   ThirdPerson.enabled = false;
}

function Update(){
if(Input.GetButton("Tab") && Active == false)
{
   FristPerson.enabled = false;
   ThirdPerson.enabled = true;
   Active = true;
}
else if(Input.GetButton("Tab") && Active == true)
{
   FristPerson.enabled = true;
   ThirdPerson.enabled = false;
   Active = false;
}