RegisterDragDrop failure with unknown result.

In my call to RegisterDragDrop I get 800700E, which does not match any of the constants expected from that function. What
does that result mean, and why does my attempt to register the treeview fail?

Thanks,
Jon

unit oledrop1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ComCtrls, ActiveX;

type
  TForm1 = class(TForm)
    TreeView1: TTreeView;
    StatusBar1: TStatusBar;
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

  TDnD = class(TInterfacedObject, IDropTarget)
  private
    FControl: TWinControl;
  public
    function DragEnter(const dataObj: IDataObject; grfKeyState: Longint;
      pt: TPoint; var dwEffect: Longint): HResult; stdcall;
    function DragOver(grfKeyState: Longint; pt: TPoint;
      var dwEffect: Longint): HResult; stdcall;
    function DragLeave: HResult; stdcall;
    function Drop(const dataObj: IDataObject; grfKeyState: Longint; pt: TPoint;
      var dwEffect: Longint): HResult; stdcall;
  end;

var
  Form1: TForm1;
  DnD: TDnD;

implementation

{$R *.dfm}

{ TDnD }

function TDnD.DragEnter(const dataObj: IDataObject; grfKeyState: Integer;
  pt: TPoint; var dwEffect: Integer): HResult;
begin
  TForm1(FControl.Owner).StatusBar1.SimpleText := 'Drag Enter';
end;

function TDnD.DragLeave: HResult;
begin

end;

function TDnD.DragOver(grfKeyState: Integer; pt: TPoint;
  var dwEffect: Integer): HResult;
begin

end;

function TDnD.Drop(const dataObj: IDataObject; grfKeyState: Integer;
  pt: TPoint; var dwEffect: Integer): HResult;
begin

end;

procedure TForm1.FormCreate(Sender: TObject);
var
  H: DWord;
  S: string;
begin
  DnD := TDnD.Create;
  DnD.FControl := TreeView1;
  H := RegisterDragDrop(TreeView1.Handle, DnD);

  if H = S_OK then
    S := 'S_OK' else
  if H = DRAGDROP_E_INVALIDHWND then
    S := 'DRAGDROP_E_INVALIDHWND' else
  if H = DRAGDROP_E_ALREADYREGISTERED then
    S := 'DRAGDROP_E_ALREADYREGISTERED' else
  if H = E_OUTOFMEMORY then
    S := 'E_OUTOFMEMORY'
  else
    S := IntToHex(H, 8);

  StatusBar1.SimpleText := S;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  DnD.Free;
end;

end.