SelectedIndexChanged not firing when MasterPageFile is used

I have a small demo aspx file that demonstrates a gridview which, when clicking on a row will display information about the row that was clicked on.  This demo works, but as soon as I attempt to add a MasterPagefile, it no longer works.  The GridView_SelectedIndexChanged does not fire.  any help would be appreciated!
Here is the aspx that works:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default5.aspx.cs" Inherits="Default5" EnableEventValidation="false" %>
<!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></title> ...