Store Instantiated Object script reference

I'm instantiating an enemy object and then want to get it's script reference and store it in a list, but I keep getting NullReferenceException: Object reference not set to an instance of an object Though it successfully gets the component, as the test function runs correctly.

 private List<Unit> enemyTeam;    
 
 Vector3 enemyPosition = new Vector3(5 + x, 1 - 1.2f*i, 0f);
 GameObject enemy = enemies[Random.Range (0, enemies.Length)];
 GameObject enemyInstance = Instantiate(enemy, enemyPosition, Quaternion.identity) as GameObject;
 Unit enemyUnit = enemyInstance.GetComponent<Unit> ();
 enemyUnit.test ();
 enemyTeam.Add (enemyUnit);

The error is produced by the last line. What I'm doing wrong? Sorry if this is a stupid mitake, I'm very new to Unity, I would also love to find a mentor :)