Checking the checkbox generates the error described below:

<%@ Page Language="C#" %>

<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<script runat="server">
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!X.IsAjaxRequest)
        {
            this.Store1.DataSource = new object[]
            {
                new object[] { 1, 4 },
                new object[] { 2, 5 },
                new object[] { 3, 6 }
            };
            this.Store1.DataBind();
        }
    }
</script>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Ext.NET Example</title>

    <script type="text/javascript">
        var somaCheckPagar = function () {
            var grd = this;
            var gridSel = grd.getSelection();
            var count = 0;
            for (var i = 0; i < gridSel.length; i++) {
                var num = gridSel[i].data.indexSum[0];
                num = parseInt(num);
                count += num;
            }
            alert(count);
        };

    </script>
</head>
<body>
    <form runat="server">
        <ext:ResourceManager runat="server" />
        <ext:GridPanel ID="GridPanel1" runat="server" AutoHeight="true">
            <Store>
                <ext:Store ID="Store1" runat="server">
                    <Model>
                        <ext:Model runat="server" AutoDataBind="true">
                            <Fields>
                                <ext:ModelField Name="test1" />
                                <ext:ModelField Name="test2" />
                            </Fields>
                        </ext:Model>
                    </Model>

                    <Reader>
                        <ext:ArrayReader />
                    </Reader>
                </ext:Store>
            </Store>
            <ColumnModel runat="server">
                <Columns>
                    <ext:Column runat="server" Header="Test1" DataIndex="test1" />
                    <ext:Column runat="server" Header="Test2" DataIndex="test2" />
                </Columns>
            </ColumnModel>
            <SelectionModel>
                <ext:CheckboxSelectionModel runat="server" Mode="Multi">
                    <Listeners>
                        <SelectionChange Fn="somaCheckPagar()" />

                    </Listeners>
                </ext:CheckboxSelectionModel>
            </SelectionModel>
        </ext:GridPanel>
    </form>
</body>
</html>
Uncaught TypeError: s.apply is not a function
at F.fire (ext.axd?v=4.2.0:19)
at F.doFireEvent (ext.axd?v=4.2.0:19)
at F.fireEventArgs (ext.axd?v=4.2.0:19)
at F.fireEvent (ext.axd?v=4.2.0:19)
at F.maybeFireSelectionChange (ext.axd?v=4.2.0:19)
at F.maybeFireSelectionChange (ext.axd?v=4.2.0:19)
at F.doMultiSelect (ext.axd?v=4.2.0:19)
at F.doSelect (ext.axd?v=4.2.0:19)
at F.selectWithEventMulti (ext.axd?v=4.2.0:19)
at F.selectWithEvent (ext.axd?v=4.2.0:19)