What wrong with this (Subproperty with TPersistent)?

I want to define property Range on my component with 2 subproperty; StartRange and EndRange, thus I defined it using the TPersistent descendant.....

unit MyUnit;

interface

uses
  Windows, Messages, SysUtils, Classes, WinInet;
  TpgHttpRange = class(TPersistent)
  private
    FStartRange: integer;
    FEndRange: integer;
  public
    procedure Assign(Source: TPersistent); override;
  published
    property StartRange : integer read FStartRange write FStartRange default 0;
    property EndRange : integer read FEndRange write FEndRange default 0;
  end;

TpgHttpDownLoad = class(TComponent)
  private
     //---others----....
     FRange : TpgHttpRange;
     // ---others-----
  published
     property Range: TpgHttpRange read FRange write
FRange;
     //------others-----
  end;

Implementation

procedure TpgHTTPRange.Assign(Source: TPersistent);
begin
    if Source is TpgHttpRange then
        with Source as TpgHttpRange do
        begin
            StartRange := Self.FStartRange;
            EndRange := Self.FEndRange;
        end
    else
        inherited Assign(Source);
end;

//---others------
end.

After registering, on object inspector when I click on
[+] before the Range (which it should expanded and dislayed subproperties), the exception 'Unable to expand'
was shown. What wrong with this implementation.....