Project Project2.exe raised exception class EAccessViolation with message 'Access violation at address 00467ECA in modul

unit Unit1;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, Grids, StdCtrls,ComObj;
 
type
  TForm1 = class(TForm)
    Button1: TButton;
    StringGrid1: TStringGrid;
    Label1: TLabel;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
 
var
  Form1: TForm1;
 
implementation
 
{$R *.dfm}
 
procedure TForm1.Button1Click(Sender: TObject);
var xl: Variant;
s:string ;
k,i,n:integer;
begin
stringgrid1.Visible:=true;
xl := CreateOleObject('Excel.Application');
xl.DisplayAlerts := True;
xl.Workbooks.Open('s');
k:=0;
n:=0;
i:=1;
while k=0 do begin
s:=xl.Cells[2,i];
if s[1] in ['А'..'Я', 'а'..'я', 'A'..'Z', 'a'..'z'] then begin    {Ошибка тут}
n:=n+1;
i:=i+7;
end
else
k:=-1;
end;
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
stringgrid1.Visible:=false;
stringgrid1.Cells[0,0]:= 'Фио Ученика';
stringgrid1.Cells[1,0]:= '% сданых зачетов';
stringgrid1.Cells[2,0]:= '% сданых экзаменов';
stringgrid1.Cells[3,0]:= 'Кол-во долгов';
end;
end.

procedure TForm1.Button1Click(Sender: TObject);
var xl: Variant;
s:string ;
k,i,n:integer;
begin
stringgrid1.Visible:=true;
xl := CreateOleObject('Excel.Application');
xl.DisplayAlerts := True;
xl.Workbooks.Open('s.xlsx');
k:=0;
n:=0;
i:=1;
while k=0 do begin
s:=xl.Cells[2,i];
if length(s)=0 then
k:=-1
else
if s[1] in ['А'..'Я', 'а'..'я', 'A'..'Z', 'a'..'z'] then begin    {Ошибка тут}
n:=n+1;
i:=i+7;
end;
end;
stringgrid1.Cells[0,1]:= inttostr(n);
end;
end.