C#

Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

1st page

//event handler
protected void ViewCustomer(object sender, EventArgs e)
    {
        Session["Selection"] = GridView1.SelectedDataKey.Value
        Response.Redirect("~/Master/SelectionBetweenTwoPagesPageTwo.aspx");
    }

//source code 

<asp:gridview id="GridView1" runat="server" autogeneratecolumns="False" xmlns:asp="#unknown">
            CellPadding="4" DataKeyNames="CustomerID" DataSourceID="SqlDataSource1" 
            ForeColor="#333333" GridLines="None" Height="139px" 
            OnSelectedIndexChanged="ViewCustomer" Width="448px">
            <footerstyle backcolor="#990000" font-bold="True" forecolor="White" />
            <rowstyle backcolor="#FFFBD6" forecolor="#333333" />
            <columns>
                <asp:boundfield datafield="CustomerID" headertext="CustomerID" readonly="false">
                    SortExpression="CustomerID" />
                <asp:boundfield datafield="CompanyName" headertext="CompanyName">
                    SortExpression="CompanyName" />
                <asp:boundfield datafield="ContactName" headertext="ContactName">
                    SortExpression="ContactName" />
                <asp:boundfield datafield="ContactTitle" headertext="ContactTitle">
                    SortExpression="ContactTitle" />
                <asp:hyperlinkfield navigateurl="~/Master/SelectionBetweenTwoPagesPageTwo.aspx">
                    Text="View"  />
            </asp:hyperlinkfield></asp:boundfield></asp:boundfield></asp:boundfield></asp:boundfield></columns>
            <pagerstyle backcolor="#FFCC66" forecolor="#333333" horizontalalign="Center" />
            <selectedrowstyle backcolor="#FFCC66" font-bold="True" forecolor="Navy" />
            <headerstyle backcolor="#990000" font-bold="True" forecolor="White" />
            <alternatingrowstyle backcolor="White" />
        </asp:gridview>
        <asp:sqldatasource id="SqlDataSource1" runat="server" xmlns:asp="#unknown">
            ConnectionString="<%$ ConnectionStrings:NORTHWNDConnectionString %>" 
            SelectCommand="SELECT [CustomerID], [CompanyName], [ContactName], [ContactTitle] FROM [Customers]">
        </asp:sqldatasource>


//2nd page


    protected void Page_Load(object sender, EventArgs e)
    {
        if (Session["Selection"] != null)
        {
            int customerid = (int)Session["Selection"];
        }

    }