'GridView1' fired event PageIndexChanging which wasn't handled

I am using a gridview and I want to use paging. I have already set allow paging to true and page size to 5. I can see the numbers at the base of my gridview, but when i click on a number to move to respective page, it throws an error saying:

The GridView 'GridView1' fired event PageIndexChanging which wasn't handled.

Code:

 <asp:GridView ID="GridView1" runat="server" CellPadding="5" 
    AutoGenerateColumns="False" AllowPaging="True" DataKeyNames="contact_id" 
    onrowcancelingedit="GridView1_RowCancelingEdit" 
    onrowediting="GridView1_RowEditing" onrowupdating="GridView1_RowUpdating" 
    PageSize="5">
    <Columns>
        <asp:TemplateField HeaderText="contact_id">
            <ItemTemplate>
                <asp:Label ID="Label3" runat="server" Text='<%# Eval("contact_id") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="name">
            <ItemTemplate>
                <asp:Label ID="Label4" runat="server" Text='<%# Eval("name") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="address">
            <ItemTemplate>
                <asp:Label ID="Label5" runat="server" Text='<%# Eval("address") %>'></asp:Label><br />
                <asp:Label ID="Label6" runat="server" Text='<%# Eval("city") %>'></asp:Label><br />
                 <asp:Label ID="Label7" runat="server" Text='<%# Eval("state") %>'></asp:Label><br />
           <asp:Label ID="Label8" runat="server" Text='<%# Eval("pincode") %>'></asp:Label>

            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="email">
            <ItemTemplate>
                <asp:Label ID="Label9" runat="server" Text='<%# Eval("email") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="mobile">
            <ItemTemplate>
                <asp:Label ID="Label10" runat="server" Text='<%# Eval("mobile") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="context">
            <ItemTemplate>
                <asp:Label ID="Label11" runat="server" Text='<%# Eval("context") %>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="status">
            <ItemTemplate>
                <asp:Label ID="Label12" runat="server" Text='<%# Eval("status") %>'></asp:Label>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:DropDownList ID="DropDownList1" runat="server">
                <asp:ListItem>PENDING</asp:ListItem>
                <asp:ListItem>OK</asp:ListItem>
                </asp:DropDownList>
            </EditItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Edit" ShowHeader="False">
            <ItemTemplate>
                <asp:LinkButton ID="LinkButton1" runat="server" CausesValidation="False" 
                    CommandName="Edit" Text="Edit"></asp:LinkButton>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:LinkButton ID="LinkButton1" runat="server" CausesValidation="True" 
                    CommandName="Update" Text="Update"></asp:LinkButton>
                &nbsp;<asp:LinkButton ID="LinkButton2" runat="server" CausesValidation="False" 
                    CommandName="Cancel" Text="Cancel"></asp:LinkButton>
            </EditItemTemplate>
            <ItemStyle CssClass="button" />
        </asp:TemplateField>
    </Columns>
    <PagerStyle HorizontalAlign="Left" VerticalAlign="Middle" />
</asp:GridView>

You will have to handle the PageIndexChanging event for the grid

Something like

protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
    GridView1.PageIndex = e.NewPageIndex;
    //Bind grid

}