NullReferenceException from transform.parent.gameObject

The following code is being used in an RTS style game to detect when a unit comes within a certain distance of a crate to allow them to ‘pick up’ objects. However, every time a unit enters the collider around the object the script is attached to, it merely throws up a NullReferenceException and nothing happens (the message sent out should stop the light attached to the object and destroy it).

using UnityEngine;
using System.Collections;

public class PickupOnCollider : MonoBehaviour {

	void OnTriggerEnter (Collider other)
	{
	GameObject enteringObject = other.transform.parent.gameObject;
		Debug.Log("Object entering collider...");
		if (enteringObject.light != null)
		{
			if (enteringObject.light.tag == "Allied")
			{	
				BroadcastMessage("OnPickup");
				Debug.Log("Item picked up.");
			}
		}
	}
}

I presume that not everything that enters this trigger has a parent? Are they just top level objects?