You must be using some <%# %> or <%= %> tag or controls like textbox etc in the GridView. Include this code in the page. You need not to write code in the function. You need to add the blank overridden functiononly. I am giving you the C# code, just change it to VB for you.
#region VerifyRenderingInServerForm
/// <summary>
/// This is function only used for the Export to Excel function in Master Pages when using the Javascript client side validation using '<%= %> '
/// </summary>
/// <param name="control"></param>
public override void VerifyRenderingInServerForm(Control control)
{
// Confirms that an HtmlForm control is rendered for the specified ASP.NET server control at run time.
}
#endregion
Hope it helps you.