Undefined or null refrence with collListItem

function readItems(resultpanel) {
    var clientContext;
    var oWebsite;
    var oList;
    var camlQuery;

    clientContext = new SP.ClientContext.get_current();
    oWebsite = clientContext.get_web();
    oList = oWebsite.get_lists().getByTitle("Announcements");
    camlQuery = new SP.CamlQuery();
    camlQuery.set_viewXml(
        '<View><Query><Where><Geq><FieldRef Name=\'ID\'/>' +
        '<Value Type=\'Number\'>1</Value></Geq></Where></Query>' +
        '<RowLimit>10</RowLimit></View>'
    );
    this.collListItem = oList.getItems(camlQuery);

    clientContext.load(this.collListItem);
    clientContext.executeQueryAsync(
        Function.createDelegate(this, successHandler),
        Function.createDelegate(this, errorHandler)
    );

    function successHandler() {
        var listItemInfo;
        var listItemEnumerator;

        listItemEnumerator = this.collListItem.getEnumerator();

        listItemInfo = "";
        while (listItemEnumerator.moveNext()) {
            var oListItem;
            oListItem = listItemEnumerator.get_current();
            listItemInfo += "ID: " + oListItem.get_id() + "<br/>" +
                "Title: " + oListItem.get_item("Title") + "<br/>" +
                "Body: " + oListItem.get_item("Body") + "<br/>";
        }

        resultpanel.innerHTML = listItemInfo;
    }