@Override
    public List<Selectable> nodes() {
        List<Selectable> selectables = new ArrayList<Selectable>();
        for (Element element : getElements()) {
            List<Element> childElements = new ArrayList<Element>(1);
            childElements.add(element);
            selectables.add(new HtmlNode(childElements));
        }
        return selectables;
    }
Only document can be select
See: https://github.com/code4craft/webmagic/issues/113

@param elementIterator elementIterator
@return element element
webmagic-core/src/main/java/us/codecraft/webmagic/selector/HtmlNode.java