Object reference error question

I'm still new to C, and am probably missing something basic. I have this script that keeps giving me this error: NullReferenceException: Object reference not set to an instance of an object ParticleCollide.OnParticleCollision (UnityEngine.GameObject other) (at Assets/Scripts/ParticleCollide.cs:35)

It compiles, and lets me run it, but everytime a collision occurs, I get the error above. I Bolded line 35 that's throwing the error.

using UnityEngine; using System.Collections;

public class ParticleCollide : MonoBehaviour { public ParticleSystem ps; GameObject GC;

void Start () {

 GC = GameObject.Find ("GrabberCube");
 ps = GetComponent<ParticleSystem>();

}

 void OnParticleCollision(GameObject other){
 ParticleSystem.Particle[] particles = new ParticleSystem.Particle[ps.particleCount];

 int num = ps.GetParticles (particles);
 //Debug.Log("found "+num+"of particles");
 for (int i=0; i<num; i++) {
 //Debug.Log ("position " + particles [i].position.z);

 **if (particles [i].position.z == GC.transform.position.z)**
     particles [i].velocity -= Vector3.up * -7.5f;
     particles [i].lifetime = 0.2f;
     Debug.Log ("Particle grabbed.");
     }
     ps.SetParticles(particles,num);
 }

}