Need help with an error message, Someone help?

I was trying to make a script that would allow my character to climb ladders. I followed a youtube video and when I tried it out an error message came up. The error message reads:

" NullReferenceException: Object reference not set to an instance of an object
LadderClimbing.OnTriggerEnter (UnityEngine.Collider col) (at Assets/LadderClimbing.js:17)
"
The script is in Javascript and I have pasted it below, please can someone tell me what the problem is? Thank you.

#pragma strict

var ChController : Transform;
var inside : boolean = false;
var heightfactor : float = 3.2;

private var FPSInput : FPSInputController;

function start () {

FPSInput = GetComponent(FPSInputController);

}

function OnTriggerEnter(col : Collider) {
if(gameObject.collider.tag == “Ladder”) {
FPSInput.enabled = false;
inside = true;

}

}

function OnTriggerExit(Col : Collider) {
if(gameObject.collider.tag == “Ladder”) {
FPSInput.enabled = true;
inside = false;

}

}

function update() {
if(inside == true && Input.GetKey(KeyCode.W)){
ChController.transform.position += Vector3.up / heightfactor;

}

}

  1. Please format your code using 10101 button
  2. your if(gameObject.collider…) should be if(col.collider…)

and capitalize Start(), and Update()

By the way, im not sure if anyone will see this but could you explain to me why the change "if (col.collider)"works?