Label an array of buttons with an array of strings

Hello,

I'm trying to label an array of buttons using an array of strings, but I get an error message saying: "Ambiguous reference 'Button': UnityEngine.GUI.Button(UnityEngine.Rect, UnityEngine.GUIContent), UnityEngine.GUI.Button(UnityEngine.Rect, UnityEngine.Texture), UnityEngine.GUI.Button(UnityEngine.Rect, String)."

I've looked through some answers here but could not solve my problem. What I've got so far is:

 var estCas : Array = new Array[9];
 function OnGUI () {
     for (var i = 0; i <= 8; i = i++){
         if (GUI.Button (Rect (posX, posY, size, size), estCas[i])) {
         }
     }
 }

Funny is that if I concatenate estCas[i] with an empty string it works, well, somewhat, unity crashes a couple seconds later and the buttons won't even appear.

Any help will be realy, realy apreciated!

*Edit (Adding full code)

What I'm trying to do is simple a Tic Tac Toe game as I'm begining with programming.

 var cell : Array = new Array[9];
 var cellEst : Array = new Array[9];
 var turn : boolean; // false = X; true = O
 
 function Start () {
     for (var i = 0; i <= 8; i = i + 1){
         cellEst[i] = ""; //Gives an empty string that can later be changed to X or O
     }
 }
 
 function OnGUI () {
     for (var i = 0; i <= 8; i = i + 1){

             //Arranges the buttons into an tic tac toe board
         var posX : int = 0;
         var posY : int = 0;
         var size : float = 100;
         
         if (i == 1 || i == 4 || i == 7) {
             posX = size;
         }
         
         if (i == 2 || i == 5 || i == 8) {
             posX = size * 2;
         }
         
         if (i == 3 || i == 4 || i == 5) {
             posY = size;
         }
         
         if (i == 6 || i == 7 || i == 8) {
             posY = size * 2;
         }
         
         if (GUI.Button (Rect (posX, posY, size, size), cellEst[i]) && cellEst[i] == "") {
             if (turn == false) {
             }
             
             else {
             }
             
             turn = !turn; //Changes player
         }
     }
 }

FIXED CODE

 var estCas : String = new String[9]; //Don't forget to add a value to the called index
 function OnGUI () {
     for (var i = 0; i <= 8; i = i + 1){ //Unity crashed when I used i++
         if (GUI.Button (Rect (posX, posY, size, size), estCas[i])) {
         }
     }
 }

Thanks a lot to Dave A, Eric5h5 and Owen Reynolds!!!