Problem accessing public method of script attached to gameobject.

Hello. I'm trying to access a public method from a script attached to a gameobject. Whenever I try to run the code inside of update in GameManager I get a NullReferenceException: Object reference not set to an instance of object. I'm not really sure whats going on here. Anyone have an idea?

public class GameManager : MonoBehaviour {

 private UnitManager unitMan;

 // Use this for initialization
 void Start () {
         ...
     unitMan = new UnitManager( playerPos, aiPos, AIUnitPrefab, UnitPrefab );

             ...
 }
 
 // Update is called once per frame
 void Update () {
     unitMan.curUnit.GetComponent<Unit>().updatePos();
 }

}

public class UnitManager { public List playerUnits = new List(); public List aiUnits = new List(); public Transform curUnit; public int test;

 public UnitManager( List<Vector3> playerPos, List<Vector3> aiPos, Transform UnitPrefab, Transform AIUnitPrefab )
 {
     genUnits( playerPos, aiPos, UnitPrefab, AIUnitPrefab );

     curUnit = playerUnits[0];
 }

 private void genUnits( List<Vector3> playerPos, List<Vector3> aiPos, Transform UnitPrefab, Transform AIUnitPrefab )
 {
     for (int i = 0; i < playerPos.Count; i++) {
         playerUnits.Add (Transform.Instantiate (UnitPrefab));
         playerUnits[i].position = playerPos[i];
     }

     for (int i = 0; i < aiPos.Count; i++) {
         aiUnits.Add (Transform.Instantiate (AIUnitPrefab));
         aiUnits[i].position = aiPos[i];
     }
 }

}