NullReferenceException question.

void OnTriggerEnter(Collider collider)
if (collider.gameObject.tag == “bullet”)

Console says collider is a null reference. How come? All objects’ colliders and triggers are enabled. How could an active collider be null? Thanks in advance.

Edit : My manager variable:

	GameObject manager;

	// Use this for initialization
	void Start () {
		manager = GameObject.Find("GameManager");

Because you’ve spelled it colllider.

Thanks, everyone. I got rid of it by simply assigning value to manager variable in Awake() method.