请delphi大侠帮在下瞧一下..................

以下是我最近刚刚编的小程序(是照书抄的),但老run不了,出现错误:
编的程序是使用鼠标拖曳的形式将几个认证的课程从一个listbox移至另一个listbox.
请高人指点,在下乃初学者,遇到此难题老是心烦意乱,在此静侯.
unit drag;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, Buttons, ExtCtrls;type
  TForm1 = class(TForm)
    Label1: TLabel;
    Label2: TLabel;
    ListBox1: TListBox;
    Label3: TLabel;
    ListBox2: TListBox;
    Bevel1: TBevel;
    BitBtn1: TBitBtn;
    procedure listbox2dragover(sender,source:tobject;x,y:integer;state
    :tdragstate;var accept:boolean);
    procedure listbox2dragdrop(sender,source:tobject;x,y:integer);
    procedure bitbtnclick(sender:tobject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;var
  Form1: TForm1;
  index:integer;
implementation{$R *.dfm}
procedure listbox2dragover(sender,source:tobject;x,y:integer;state
    :tdragstate;var accept:boolean);
begin
accept:=(source is tlistbox);
end;
procedure tform1.listbox2dragdrop(sender,source:tobject;x,y:integer)
begin
  index:=tlistbox(source).ItemIndex;
  if index<>1 then begin
     tlistbox(sender).Items.add(tlistbox(source).Items[index]);
     tlistbox(source).Items.delete(index);
   end;
  end;
  procedure tform1.bitbtn1click(sender:tobject);
  begin
    showmessage(format('您已通过%d科认证课程',[listbox2.items.count]);
  end;
end.