Instantiate method causes Unity to crash

I was making a spawn system for my top-down shooter, when I came across a problem. When pressing play in the editor, Unity would freeze up (at first I thought it was just loading), but when I checked in the task manager, it said Unity was not responding. Here's my code.

 public Wave[] waves;
 public Enemy enemy;

 Wave currentWave;
 int currentWaveNumber;

 int enemiesRemaingToSpawn;
 float nextSpawnTime;


 [System.Serializable]
 public class Wave
 {
     public int enemyCount;
     public float timeBetweenSpawns;
 }

 void Start()
 {
     NextWave();
 }

 void Update()
 {
     if(enemiesRemaingToSpawn > 0 && Time.time > nextSpawnTime)
     {
         enemiesRemaingToSpawn --;
         nextSpawnTime = Time.time + currentWave.timeBetweenSpawns;

         Enemy spawnedEnemy = Instantiate(enemy, Vector3.zero, Quaternion.identity) as Enemy;
     }
 }

 void NextWave()
 {
     currentWaveNumber ++;
     currentWave = waves[currentWaveNumber - 1];

     enemiesRemaingToSpawn = currentWave.enemyCount;
 }

The only way I could fix the problem was by commenting out the Instantiate method. This is of course not possible because then there wouldn't be any enemies.

This code is from a tutorial on Youtube.