NullReferenceException inside clone script

I keep getting the same error on this and it’s had me stumped for hours. Essentially I instantiate an object called “unit”, which has a script called movement which instantiates “head” (in the start function). Except that it doesn’t instantiate it, the following code gives this.

public class Movement : MonoBehaviour { //Movement is the behaviour which allows a unit to move
	public Transform HeadPrefab;
	public Transform Block;
	public int MaxSize = 5;
	Transform head;
	List<Transform> blockList = new List<Transform>();
	void Start () {
		Vector3 headPosition = new Vector3(0,5,-11);	
		head = Instantiate (HeadPrefab, headPosition, Quaternion.identity) as Transform;

gives: NullReferenceException: Object reference not set to an instance of an object.
at runtime, when compiled there are no errors.

The unit object is instantiated by a script called GameArena and is as follows

	public Transform unit;
	public Transform enemy;
	Transform clone;
	void Start () {
		Vector3 unitPosition = new Vector3(0,5,-11);
		Vector3 enemyPosition = new Vector3(10,5,-11);		
		clone = Instantiate (unit, unitPosition, Quaternion.identity) as Transform;
		Instantiate (enemy, enemyPosition, Quaternion.identity);

again no errors at all on this and it reports to the console the correct position of 0.

I’m confused that I seem to be doing the same things twice but it only works one time, what’s different in “Movement”?

I’ve created a sphere prefab and plugged it into the first script in inspector for both transform variables. Everything is working without errors. Check your prefabs