Code works in editor, however not in build... C#

As the title says, I have code below that will work in the editor however when it comes to build, it causes an error somewhere after the instantiation which is located in the for loop within the coroutine Fire. On FireMode.auto, _3rd and _2rd they only instantiate once meaning that the loop has a forced break if that helps at all.

 using UnityEngine;
 using System.Collections;
 
 public class FireParticles : MonoBehaviour {
 
     public enum FireMode { Single, Auto, _3rd, _2rd}
     public FireMode fireMode;
     public int RPM = 1000;
 
     int bumpRand = 200;
     float RPS;
     int burst;
     bool cycle;
 
     int fireAmt;
 
     public Object particles;
 
     void Update () {
         if(fireMode == FireMode._3rd)
         {
             burst = 3;
         }
         else if(fireMode == FireMode._2rd)
         {
             burst = 2;
         }
         else
         {
             burst = 1;
         }
         if ((Input.GetMouseButtonDown(0) || ((fireMode == FireMode.Auto && Input.GetMouseButton(0))) && !cycle)) {
             cycle = true;
             RPS = (1f / (RPM / 60f));
             StartCoroutine(Fire());
 
         }
     }
 
     IEnumerator Fire ()
     {
         for (int i = 0; i < burst; i++)
         {
             Instantiate(particles, transform.position, transform.rotation);
             fireAmt = fireAmt + 1;  // debug
             print(fireAmt);  // debug
             yield return new WaitForSeconds(RPS / Time.timeScale);
         }
         cycle = false;
     }
 
 }
 

This is the error that pops up; usually when I see a NullRefrenceException I know what to kinda look for, but this is one has me confused. Especially when it works fine in editor but not build.

 NullRefrenceException: Object refrence not set to an instance of an object
 > FireParticles+<Fire>c_Iterator1.MoveNext()

Any help would be very much appreciated, I'm running Unity 5.3.1 if that has to do with anything and if anyone does have an answer an explanation to where I've gone wrong would be nice.

Cheers,