Error message when Shuriken particles play...

I got my Shuriken Particle System to play on a button press, and it works exactly as i want it to.

But i get an error message i've not seen before when i do so:

UpdateParticle system should not happen on disables GO

I assume GO stands for Game Object.

I don't want problems further down the line, so can anyone tell me why i get the error? Code below :)

 var beam : ParticleSystem;
 
 function Start(){
 
 beam = GetComponent(ParticleSystem);
 beam.Stop();
 beam.Clear();
 
 }
 
 function Update () {
 
 if (Input.GetMouseButtonDown(0)){
 
 beam.Play();
 
 }
 
 if (Input.GetMouseButtonUp(0)){
 
 beam.Stop();
 beam.Clear();
 
 }
 
 }

Thanks, Tom :)