BX.PopupWindow is not a constructor

popupWindow = new BX.PopupWindow('some_block', window.body, {
        autoHide : true,
        offsetTop : 1,
        offsetLeft : 0,
        lightShadow : true,
        closeIcon : true,
        closeByEsc : true,
        overlay: {
            backgroundColor: 'black', opacity: '50'
        }
});