NullReferenceException: Object reference not set to an instance of an object

I have come across a lot of people that have had this error, however I haven't been able to fix the error myself. I am simply trying to have an enemy collider deal damage to my player.

Player:

 public class PlayerHealth : MonoBehaviour {
 
     public int maxHealth = 100;
     private int currentHealth;
 
     void Awake()
     {
         currentHealth = maxHealth;
     }
 
     void Start () 
     {
         print (currentHealth);
     }
 
     void OnCollisionEnter(Collision collision)
     {
         if (collision.gameObject.tag == "Enemy")
         {
             currentHealth -= GetComponent<EnemyStats> ().enemyDamage;
             //currentHealth -= GetComponent<EnemyStats> ().enemyDamage;
             Debug.Log("You have been hit!" + currentHealth);
         }
     }
 }

And for the enemy:

public class EnemyStats : MonoBehaviour {

 public int maxHealth = 100; 
 private int currentHealth;
 public int enemyDamage = 10;
 public int armorRating = 10;

 void Awake()
 {
     currentHealth = maxHealth;
 }

 public void Damage(int gunDamageAmount)
 {
     currentHealth -= gunDamageAmount - armorRating;

     if (currentHealth <= 0f) 
     {
         gameObject.SetActive (false);
         Debug.Log ("Enemy killed!");
     }
 }

}