null array?

I have been working all day and normally i figure out how to do something after being stuck on it for a while, its amazing ive gotten this far to me but now im stuck, ive been searching up to why this happens, cant find anything, the A appears to be null, i tried removing 0 and making 27 that still made the A or something null, i also wrote a print thing before and that printed out everything but A which would have been null. any help? using UnityEngine; using System.Collections; using System.Collections.Generic;

 public class arrayRandom : MonoBehaviour {
 
     // Use this for initialization
     void Start () {
 
     }
     
     // Update is called once per frame
     private void Update () {
         if (Input.GetKeyDown (KeyCode.G)) {
 
             string[] letters = new string[27];
             letters [0] = "A";
                 letters [1] = "B";
                 letters [3] = "C";
                 letters [4] = "D";
                 letters [5] = "E";
                 letters [6] = "F";
                 letters [7] = "G";
                 letters [8] = "H";
                 letters [9] = "I";
                 letters [10] = "J";
                 letters [11] = "K";
                 letters [12] = "L";
                 letters [13] = "M";
                 letters [14] = "N";
                 letters [15] = "O";
                 letters [16] = "P";
                 letters [17] = "Q";
                 letters [18] = "R";
                 letters [19] = "S";
                 letters [20] = "T";
                 letters [21] = "U";
                 letters [22] = "V";
                 letters [23] = "W";
                 letters [24] = "X";
                 letters [25] = "Y";
                 letters [26] = "Z";
             System.Random rnd = new System.Random ();
             //print (letters [rnd.Next (0, 26)]);
             GetComponent<GUIText> ().text = letters [rnd.Next (0, 26)];
 
 
         }
     }
 }