audio.IsPlaying dosn't work

Hello,

I wrote this script that is supposed to play a sound when you click a door. The problem is that it must prevent the sound to play if the player spams the door, but it doesn't work. When I click the object, no matter if a sound is already playing, it plays it again... Any help?

 #pragma strict
 var soundToPlay : AudioClip[];
 var knocks : int;
 function Start () {
 
 }
 
 function OnMouseDown () {
     if(!audio.isPlaying)
     {
         var index : int = Random.Range(0,12);
         audio.clip = soundToPlay[index];
         audio.PlayOneShot(soundToPlay[index]);
         knocks ++;
     }
 }