Question beginner problem Help Needed

I'm makeing a unity fps game and I have been trying to implement enemy healthbars above their head. I keep gettting the error
NullReferenceException: Object reference not set to an instance of an object
Target.Start () (at Assets/scripts/Target.cs:12)
and I feel like it's probably a really easy fix but I can't figure it out. here's my Target code

using UnityEngine;
using UnityEngine.UI;
public class Target : MonoBehaviour{

public float health = 50f;
public Slider slider;


void Start()
{
slider.value = health;
}

void update()
{
slider.value = health;
}
public void TakeDamage (float ammount)
{
health -= ammount;
if(health <= 0f)
{
Die();
}
void Die()
{
Destroy(gameObject);

}
}

Code (CSharp):

1.  
2. using UnityEngine;
3. using UnityEngine.UI;
4. public class Target : MonoBehaviour{
5.  
6.     public float health = 50f;
7.     public Slider slider;
8.    
9.  
10.     void Start()
11.     {
12.         slider.value = health;
13.     }
14.  
15.     void update()
16.     {
17.         slider.value = health;
18.     }
19.     public void TakeDamage (float ammount)
20.     {
21.         health -= ammount;
22.         if(health <= 0f)
23.         {
24.             Die();
25.         }
26.         void Die()
27.         {
28.             Destroy(gameObject);
29.            
30.         }
31.     }
32.  

if you need any other code, please, just ask

 

In the Inspector, did you give the slider object to the script? I feel like that might be the object it can't find, which is why it is throwing a null pointer exception.

 

yeah I had 2 enemy objects and I forgot about one of them

 

thanks for the help man