Try to create a component with a TStrings property

I try to create a comonent
When i put the component on a form and click on nstring I get an Abstract Error.


unit Str;
interface
uses
  Windows, Messages, SysUtils, Classes, StdCtrls;
type
  TStr = class(TComponent)
  private
  protected
    astring : TStrings;
  public
    constructor Create(AOwner: TComponent);  override;
    destructor Destroy; override;
  published
    property nstring: TStrings READ astring WrITE astring;
  end;

procedure Register;

implementation

procedure Register;
begin
  RegisterComponents('Samples', [TStr]);
end;

constructor TStr.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  astring := TStrings.Create;
end;

destructor TStr.Destroy;
begin
 inherited Destroy;
end;

end.