我写的堆栈哪里错了？

Question

我自己写了一个堆栈，
type TStack=class
private
size:Integer;
container:array [1..20] of Integer;
public
constructor Create();
function Push(aNode:Integer):boolean;
function Pop(out aNode:Integer):boolean;
end;
implementationconstructor TStack.Create();
begin
size:=0;
end;function TStack.Push(aNode:Integer):boolean;
begin
 if(size>=20) then
    begin
    Exception.Create('Stack is full!');
    result:=false;
    end
   else
   begin
    container[size]:=aNode;
    Inc(size);
    result:=True;
   end;end;function TStack.Pop(out aNode:Integer):boolean;
begin
if (size<=0) then
    begin
    Exception.Create('Stack is empty');
    result:=False;
    end
    else
     begin
      aNode:=container[size];
      dec(size);
      result:=True;
     end;   end;
测试代码如下，运行后点击pop,push就会出现内存冲突，请问怎么回事？
var
  Form1: TForm1;
  S1:TStack;
procedure TForm1.Button1Click(Sender: TObject);
begin
S1.Push(StrToInt(Edit1.Text));
end;procedure TForm1.Button2Click(Sender: TObject);
var
msg:String;
node:Integer;
begin
 S1.Pop(node);
 msg:=IntTostr(node);
 Showmessage(msg);
end;