NullReferenceException trouble

Having some trouble fixing this "NullReferenceException: Object Reference not set to instance of an object". I'm sure its something silly, but I'm new to Unity and can't see how to fix it. Any help is much appreciated. Here is the relevant code, the error occurs at the line "tiles.push(floorPosition)" :

 var floorPosition : Vector3; 
 var tiles : Array; 
 
 function Grid(){ 
     floorPosition = Vector3(-1005.4, 0, -1339.9);  
     tiles.push(floorPosition); 
     var tempXPos : float; 
     var tempZPos : float; 
     var tempPos = new Vector3(tempXPos, 0, tempZPos);
     for (var i = 0; i < 141; i++){ //141 tiles in the X direction
         tempXPos = floorXPos + (100*i); 
         for (var j = 0; j < 130; j++){ //130 tiles in Z direciton
             tempZPos = floorZPos + (100*j); 
             tiles.Push(tempPos); 
         }
     }
     return tiles;
 }

Thanks