When I scroll somewhere and call store.reload() once or twice it's throw "PageMap asked for range which it does not have"

I've just add "Refresh" button to ext-4.2.0.265/examples/grid/infinite-scroll.html file
Code:
Ext.require([
    'Ext.grid.*',
    'Ext.data.*',
    'Ext.util.*',
    'Ext.grid.plugin.BufferedRenderer'
]);


Ext.onReady(function(){
    Ext.define('ForumThread', {
        extend: 'Ext.data.Model',
        fields: [
            'title', 'forumtitle', 'forumid', 'username', {
                name: 'replycount',
                type: 'int'
            }, {
                name: 'lastpost',
                mapping: 'lastpost',
                type: 'date',
                dateFormat: 'timestamp'
            },
            'lastposter', 'excerpt', 'threadid'
        ],
        idProperty: 'threadid'
    });

    // create the Data Store
    var store = Ext.create('Ext.data.Store', {
        id: 'store',
        model: 'ForumThread',
        remoteGroup: true,
        // allow the grid to interact with the paging scroller by buffering
        buffered: true,
        leadingBufferZone: 300,
        pageSize: 100,
        proxy: {
            // load using script tags for cross domain, if the data in on the same domain as
            // this page, an Ajax proxy would be better
            type: 'jsonp',
            url: 'http://www.sencha.com/forum/remote_topics/index.php',
            reader: {
                root: 'topics',
                totalProperty: 'totalCount'
            },
            // sends single sort as multi parameter
            simpleSortMode: true,
            // sends single group as multi parameter
            simpleGroupMode: true,

            // This particular service cannot sort on more than one field, so grouping === sorting.
            groupParam: 'sort',
            groupDirectionParam: 'dir'
        },
        sorters: [{
            property: 'threadid',
            direction: 'ASC'
        }],
        autoLoad: true,
        listeners: {

            // This particular service cannot sort on more than one field, so if grouped, disable sorting
            groupchange: function(store, groupers) {
                var sortable = !store.isGrouped(),
                    headers = grid.headerCt.getVisibleGridColumns(),
                    i, len = headers.length;
                
                for (i = 0; i < len; i++) {
                    headers[i].sortable = (headers[i].sortable !== undefined) ? headers[i].sortable : sortable;
                }
            },

            // This particular service cannot sort on more than one field, so if grouped, disable sorting
            beforeprefetch: function(store, operation) {
                if (operation.groupers && operation.groupers.length) {
                    delete operation.sorters;
                }
            }
        }
    });

    function renderTopic(value, p, record) {
        return Ext.String.format(
            '<a href="http://sencha.com/forum/showthread.php?t={2}" target="_blank">{0}</a>',
            value,
            record.data.forumtitle,
            record.getId(),
            record.data.forumid
        );
    }

    var grid = Ext.create('Ext.grid.Panel', {
        width: 700,
        height: 500,
        collapsible: true,
        title: 'ExtJS.com - Browse Forums',
        store: store,
        loadMask: true,
        selModel: {
            pruneRemoved: false
        },
        multiSelect: true,
        viewConfig: {
            trackOver: false
        },
        features:[{
            ftype: 'grouping',
            hideGroupedHeader: false
        }],
        // grid columns
        columns:[{
            xtype: 'rownumberer',
            width: 50,
            sortable: false
        },{
            tdCls: 'x-grid-cell-topic',
            text: "Topic",
            dataIndex: 'title',
            flex: 1,
            renderer: renderTopic,
            sortable: true
        },{
            text: "Author",
            dataIndex: 'username',
            width: 100,
            hidden: true,
            sortable: true
        },{
            text: "Replies",
            dataIndex: 'replycount',
            align: 'center',
            width: 70,
            sortable: false
        },{
            id: 'last',
            text: "Last Post",
            dataIndex: 'lastpost',
            width: 130,
            renderer: Ext.util.Format.dateRenderer('n/j/Y g:i A'),
            sortable: true,
            groupable: false
        }],
        tbar: [{
            text: 'Refresh',
            handler: function (){
                store.reload();
            }
        }],
        renderTo: Ext.getBody()
    });
});
According to my guesswork method Ext.data.Store.reload has this code
Code:
        // If buffered, we have to clear the page cache and then
        // cache the page range surrounding store's loaded range.
        if (me.buffered) {

            // So that prefetchPage does not consider the store to be fully loaded if the local count is equal to the total count
            delete me.totalCount;

            waitForReload = function() {
                if (me.rangeCached(startIdx, endIdx)) {
                    me.loading = false;
                    me.data.un('pageAdded', waitForReload);
                    records = me.data.getRange(startIdx, endIdx);
                    me.fireEvent('load', me, records, true);
                }
            };
            bufferZone = Math.ceil((me.leadingBufferZone + me.trailingBufferZone) / 2);

            // Get our record index range in the dataset
            startIdx = options.start || (count ? me.getAt(0).index : 0);
            endIdx = startIdx + (options.count || (count ? count : me.pageSize)) - 1;

            // Calculate a page range which encompasses the Store's loaded range plus both buffer zones
            startPage = me.getPageFromRecordIndex(Math.max(startIdx - bufferZone, 0));
            endPage = me.getPageFromRecordIndex(endIdx + bufferZone);

            // Clear cache (with initial flag so that any listening BufferedRenderer does not reset to page 1).
            me.data.clear(true);

            if (me.fireEvent('beforeload', me, options) !== false) {
                me.loading = true;

                // Wait for the requested range to become available in the page map
                // Load the range as soon as the whole range is available
                me.data.on('pageAdded', waitForReload);

                // Recache the page range which encapsulates our visible records
                for (i = startPage; i <= endPage; i++) {
                    me.prefetchPage(i, options);
                }
            }
        } else { ...}
endPage increase every time when I click refresh button, but according to store.purgePageCount only last pages will be presented in LruCache, so I catch this error