Iterating Treeview in SHBrowseForFolder Dialog.

I need to remove non-fixed drives in a SHBrowseForFolder dialog. I am passing the handle
to the treeview from an EnumChildProc that is called in the BrowseDialogCallBack.

type
  Params  = packed record
    LocalDrives:    bool;
    pidlRoot:       PItemIDList;
    BffInitialized: bool;
    hWndTreeCtrl:   hwnd;
    hWndEditCtrl:   hwnd;
    InitialFolder:  pchar;
    SelectedItem:   htreeitem;
    SelectedPath:   array[0..MAX_PATH] of char;
  end;

var
  BffParams:  Params;

procedure TVLocalDrives (hwnd: hwnd);
var
  hti, child:         htreeitem;
  htiText:            array[0..MAX_PATH] of char;
  TVI:                tagTVItem;
  Count, DriveType:   uint;
begin
    hti := TreeView_GetRoot(BffParams.hWndTreeCtrl);
    TVI.mask        := TVIF_HANDLE or TVIF_TEXT or TVIF_CHILDREN or TVIF_STATE or TVIF_PARAM;
    TVI.hItem       := hti;
    TVI.pszText     := htiText;
    TVI.cchTextMax  := MAX_PATH;
    TreeView_GetItem(BffParams.hWndTreeCtrl, TVI);

    child := TreeView_GetChild(BffParams.hWndTreeCtrl, hti);
    //Build TreeViewItem Structure used to retrieve text of selected item.
    TVI.mask        := TVIF_HANDLE or TVIF_TEXT or TVIF_CHILDREN or TVIF_STATE or TVIF_PARAM;
    TVI.hItem       := child;
    TVI.pszText     := htiText;
    TVI.cchTextMax  := MAX_PATH;
    TreeView_GetItem(BffParams.hWndTreeCtrl, TVI);

    While hti <> nil do
    begin
      TVI.hItem     := hti;
      TreeView_GetItem(BffParams.hWndTreeCtrl, TVI);
      DriveType := GetDriveType(pchar(leftstr(string(TVI.pszText),3)));
      if DriveType <> DRIVE_FIXED then
      begin
        //TreeView_DeleteItem(BffParams.hWndTreeCtrl, TVI.hItem);
      end;

      hti := TreeView_GetNextItem(BffParams.hWndTreeCtrl, TVI.hItem, TVGN_NEXT);
    end;
end;

This isn't working, the call:
    child := TreeView_GetChild(BffParams.hWndTreeCtrl, hti);
does not return a child.

I have verified that the the Root is 'My Computer'.

Does it look like I'm doing anything else wrong?