Trying to save/load from PlayerPrefs to Array and vice-versa

I'm trying to keep a ranking. When i test with a simple float variable it works fine, the code is this:

 var controller : Component;
 var ranking : float;
 var done : boolean;
 
 function Start () {
     controller = GetComponent("GameController");
     pegaRanking();
 }
 
 function Update () {
     if (controller.end && !done){
         rankeia(controller.time);
         setaRanking();
         showRanking();
         done = true;
     }
 }
 
 function pegaRanking(){
     if(PlayerPrefs.HasKey("lugar")) ranking = PlayerPrefs.GetFloat("lugar");
 }
 
 function setaRanking(){
     PlayerPrefs.SetFloat("lugar",ranking);
 }
 
 function rankeia(tempo : float){
     if(ranking == null){
         ranking = tempo;
         return;
     }
     if(tempo > ranking) ranking = tempo;
 }
 
 function showRanking(){
     Debug.Log(ranking);
 }

but when i make ranking into a float[], it simply doesnt work. I've been testing with the very same code, like this:

 var controller : Component;
 var ranking : float[];
 var done : boolean;
 
 function Start () {
     controller = GetComponent("GameController");
     pegaRanking();
 }
 
 function Update () {
     if (controller.end && !done){
         rankeia(controller.time);
         setaRanking();
         showRanking();
         done = true;
     }
 }
 
 function pegaRanking(){
     if(PlayerPrefs.HasKey("lugar")) ranking[0] = PlayerPrefs.GetFloat("lugar");
 }
 
 function setaRanking(){
     PlayerPrefs.SetFloat("lugar",ranking[0]);
 }
 
 function rankeia(tempo : float){
     if(ranking[0] == null){
         ranking[0] = tempo;
         return;
     }
     if(tempo > ranking[0]) ranking[0] = tempo;
 }
 
 function showRanking(){
     Debug.Log(ranking[0]);
 }

the only difference is that there's a [0] after every ranking. why does it not work?

thanks in advance