为什么紧缩地图会出错？

<P>procedure TfrmMain.NPackMapClick(Sender: TObject);
const
   packOptionArray:array[0..5] of integer=(1,2,4,8,16,32);
var
  i,j:integer;
begin
  for i:=1 to mapMain.Layers.Count do
  begin
    for j:=0 to 5 do
    begin
      if mapMain.Layers.SupportsPack(packOptionArray[j]) then
        mapMain.Layers.Pack(packOptionArray[j]);
    end;  //end j
  end; //end i
end;</P>
<P>13:31:19:Access violation at address 018FAD66 in module 'daengine.dll'. Read of address 00000000
</P>