Object reference not set to an instance of an object

Hi,
I have the following code that keeps giving me an error "System.NullReferenceException: Object reference not set to an instance of an object." on the line "btnProductA.Attributes.Add("onmouseover", "imageOn();");".  The code (giving the error) is in a content page of a master page.  How can I resolve this?

Thanks very much in advance!

Here's the code in the Content Page (which is embedded within a Master Page):
 
<asp:Content ID="Content1" ContentPlaceHolderID="ProductImages" Runat="Server">
    <table border="0" cellpadding="0" cellspacing="0">
        <tr>
            <td>
                <asp:ImageButton ID="btnProductA" runat="server" ImageUrl="images/ProductA.gif" BorderWidth="0" />
            </td>
        </tr>
        <tr>
            <td>
                  Click on product picture.
            </td>
        </tr>
    </table>
</asp:Content>
 
Here's the code behind of the Content Page:
 
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
 
public partial class customers : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Utilities.RegisterBaseUrl(this.Page);
        ImageButton btnProductA = (ImageButton)FindControl("btnProductA");
        btnProductA.Attributes.Add("onmouseover", "imageOn();");
        btnProductA.Attributes.Add("onmouseout", "imageOff();");
    }
}

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