请Invalid pointer operation 异常

请高手指点Invalid pointer operation 错误
小弟正在学习delphi的dll文件的编写,把以下一个小函数放在mytools.dll中
然后在程序中测试调用的时候能得到结果,可是也得到一个错误提示 "Invalid   pointer   operation ".请各位能帮帮我.
//IntToLZStr小函数
function   IntToLZStr(Value:integer;Digits:byte)   :   string;stdcall;
var
Res   :   string;
begin
    Res:=IntToStr(value);
    while   Length(Res) <digits   do
        Insert( '0 ',res,1);
    IntToLZStr:=Res;
end;

测试调用mytools.dll中的IntToLZStr函数的代码
unit   Unit1;

interface

uses
    Windows,   Messages,   SysUtils,   Variants,   Classes,   Graphics,   Controls,   Forms,
    Dialogs,   printers,   StdCtrls;

type
    TForm1   =   class(TForm)
        Button1:   TButton;
        Edit1:   TEdit;
        Edit2:   TEdit;
        procedure   Button1Click(Sender:   TObject);
    private
        {   Private   declarations   }
    public
        {   Public   declarations   }
    end;

var
    Form1:   TForm1;

implementation

{$R   *.dfm}

function   IntToLZStr(Value:integer;Digits:byte)   :   string;stdcall;   external   'mytools.dll ';

procedure   TForm1.Button1Click(Sender:   TObject);
var
    str:string;
begin
    str:=   trim(edit2.Text);
    edit1.Text:=IntToLZStr(strtoint(str),10);
end;

en

------解决方案--------------------
给你改了一下，我测试通过：

1、DLL：

library MyDll;

{ Important note about DLL memory management: ShareMem must be the
first unit in your library 's USES clause AND your project 's (select
Project-View Source) USES clause if your DLL exports any procedures or
functions that pass strings as parameters or function results. This
applies to all strings passed to and from your DLL--even those that
are nested in records and classes. ShareMem is the interface unit to
the BORLNDMM.DLL shared memory manager, which must be deployed along
with your DLL. To avoid using BORLNDMM.DLL, pass string information
using PChar or ShortString parameters. }

uses
SysUtils,
Classes;

function IntToLZStr(Value:integer;Digits:byte) : PChar; stdcall; export;
var
Res : String;
begin
Res:=IntToStr(value);
while Length(Res) <digits do
Insert( '0 ',res,1);
IntToLZStr:=PChar(Res);
end;

exports
IntToLZStr index 1 name 'IntToLZStr ' resident;

begin
end.
-------------------------------------------------------

2、Exe：

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, printers, StdCtrls;

type
TForm1 = class(TForm)
Edit1: TEdit;
Edit2: TEdit;
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form1: TForm1;

implementation

{$R *.dfm}