Object not set to an instance of an object?? why?

here is the code:

  var SystemAssetList : List.<String>;
             
      function GetList(){
             var AssetList : String[];
         AssetList=Directory.GetFiles(Application.dataPath+"/Bundles/");
     
         for(i=0;i<AssetList.length;i++){
                SystemAssetList.Add(AssetList[i]);
         }
     }

why it gives me this error? both arrays are String type? cross post:http://forum.unity3d.com/threads/104440-Strange-i-cannot-figure-out-this-quot-Object-reference-not-set-to-an-instance...?p=688675#post688675