Whats Wrong?

Hi, I'm trying to make an Array of game objects at Start found around an enemy using Physics.OverlapSphere, i thought this script I've done would do it, but says on the second find -

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

If I remark out

 concreteBarriers[i] = current.gameObject;

it prints out all the names and numbers up to the 16 it finds, can someone point me in the right direction please.

 function Start()
 {
     hide();
 }
 
 function hide()
 {
 
 var colliders : Collider[] = Physics.OverlapSphere (transform.position, lookForCoverRange);
 var cover : GameObject[];
 
 for (var current : Collider in colliders)
 {
     print(current.name + " Number " + i);
     cover[i] = current.gameObject;
     i++;
     }
 }