Null Reference Exception in very simple script?

I've been following a few simple tutorials so as to pick up some basics and I am now coming across this problem. Its a simple drag n drop script but I'm getting this error -

NullReferenceException: Object reference not set to an instance of an object
dragObject.OnMouseDrag () (at Assets/dragObject.cs:21)

I'm probably making a real noob error, but I just can't see what it is?

full script -

Code (CSharp):

1. public class dragObject : MonoBehaviour
2. {
3.     Vector3 dist;
4.     float posX;
5.     float posY;
6.  
7.     void OnMouseDown()
8.     {
9.         dist = Camera.main.WorldToScreenPoint(transform.position);
10.         posX = Input.mousePosition.x - dist.x;
11.         posY = Input.mousePosition.y - dist.y;
12.     }
13.  
14.     void OnMouseDrag()
15.     {
16.         Vector3 curPos = new Vector3(Input.mousePosition.x - posX, Input.mousePosition.y - posY, dist.z);
17.         Vector3 worldPos = Camera.main.ScreenToWorldPoint(curPos);
18.         transform.position = worldPos;
19.     }
20.  
21. }

Any help would be greatly appreciated, thanks !

 

Is this script attached to an object that has a Collider or GUIElement?

 

The object is a cube, just has the standard Box Collider on it

 

The script works fine for me. What version of Unity are you on? Could be a bug in your current Unity version. You may need to update to a new version or newer Patch Release

 

2017, only re-installed Unity yesterday

 

Camera.main can sometimes return null.

Code (CSharp):

1. Camera mainCam;
2.  
3. void Start()
4. {
5.    mainCam = Camera.main;
6.  
7.    if(mainCam == null)
8.       StartCoroutine(FindCamera);
9. }
10.  
11. IEnumerator FindCamera()
12. {
13.    While(mainCam == null)
14.    {
15.       mainCam = Camera.main;
16.       yield return null;
17.    }
18. }

 

OK for some reason it was Unity itself. Did a un-install and fresh re-install of Unity and all is working fine. Thanks for the help guys

 

I would avoid this.
Just make sure the camera exists at scene startup and does not get destroyed / its tag won't be changed.