Null Reference Exception after updating from 4.0 to 4.1.5 (coroutines)

UPDATE

I replicated the issue below in an empty project, confirmed it and posted it here.

I have a piece of code that works perfectly on Unity 4.0, however after I updated my unity version to 4.1.5, it is not working properly anymore.(it is also broken on 4.1.2) Precisely, I am getting a null reference exception while attemping to call a function which is a coroutine, whose type is IEnumerator. I made sure that all parameters are valid and I can access the function.

This function is being called from a javascript object.(a class instance of a GO) And the function is on a seperate object.

 function showSimpleEffect ( c : GameObject, effectName : String, duration : float, scale : Vector3 ) : IEnumerator{
 
     var simpleEffect : GameObject = Instantiate( simpleEffectPlanePrefab , c.transform.position + Vector3(0,0.4,0) , simpleEffectPlanePrefab.transform.rotation );
     simpleEffect.transform.localScale = scale;
     simpleEffect.renderer.materials[0].SetTexture("_MainTex", Resources.Load("simpleSfxTextures/" + effectName ));
     simpleEffect.transform.parent = c.transform;
 
     var rate = 1.0/duration;
     var t = 0.0;
     
     while (t < 1.0) {
     
         t += Time.deltaTime * rate;
         
         if( t <= 0.4 ){
             simpleEffect.renderer.material.SetColor ( "_Color", Color( Mathf.Lerp( 0.5, 0.5, t*2.5 ) , Mathf.Lerp( 0.5, 0.5, t*2.5 ), Mathf.Lerp( 0.5, 0.5, t*2.5 ) , Mathf.Lerp( 0.0, 1.0, t*2.5 ) ) );
         }else if( t >= 0.6 ) {
             simpleEffect.renderer.material.SetColor ( "_Color", Color( Mathf.Lerp( 0.5, 0.5, (t-0.6)*2.5 ) , Mathf.Lerp( 0.5, 0.5, (t-0.6)*2.5 ),Mathf.Lerp( 0.5, 0.5, (t-0.6)*2.5 ), Mathf.Lerp( 1.0, 0.0, (t-0.6)*2.5 ) ) );
         }
         
         yield;
     }
     
     Destroy(simpleEffect);
 
 }

This is a very simple function that shows an image above a gameobject for a number of seconds. I have to point out that if I remove the while loop, the function works properly. (Probably because it is not a coroutine anymore) I don't know what the developers of Unity changed after 4.0 but I can't figure it out by myself, any help or clue is appreciated.

 NullReferenceException
 Effect.onActivate (Boolean applyResult) (at Assets/Scripts/cardFunctions.js:2098)
 Effect.setActiveness (Boolean applyResult) (at Assets/Scripts/cardFunctions.js:1849)
 Effect.onAdd () (at Assets/Scripts/cardFunctions.js:2540)
 cardFunctions.addEffect (UnityEngine.GameObject onWhichCard, .Effect effectToAdd) (at Assets/Scripts/cardFunctions.js:2690)
 cardProperties.onPlay () (at Assets/Scripts/cardProperties.js:2887)
 mastermind+$moveCardToSlotEnemy$1164+$.MoveNext () (at Assets/Scripts/mastermind.js:1538)
 

Thanks in advance.