Crash when adding Category

Hello.

I am trying to implement a PropertyGrid in a dialog in a MDI application. When I add a category to this PropertyGrid, the application is crashing (some ASSERT failure in the MFC library)...

BOOL CChunkBuilderMain::OnInitDialog()
{
  LPCTSTR  pCategory;
  int      iCatIndex;

 CDialog::OnInitDialog();

 CRect rc;
 m_chunkParamsPlaceHolder.GetWindowRect( &rc );
 ScreenToClient( &rc );

 // create the property grid.
 if ( m_wndPropertyGrid.Create( rc, this, IDC_PROPERTY_GRID ) )
 {
  LOGFONT lf;
  GetFont()->GetLogFont( &lf );

  // create document settings category.
    CXTPPropertyGridItem* pSettings = m_wndPropertyGrid.AddCategory("Document Settings");
  .....

}