Master page error when trying to run project

Hi
I trying to run my project and I get  the following error 'lstpreferredtheme_selectedindexchanged is not a member of ASP.Masterpages_Masterpage_master
Even I've named the id of my dropdown list lstpreferredtheme which I trying to call. My code is below
<%@ Master Language="VB" CodeFile="MasterPage.master.vb" Inherits="MasterPage" %>

<!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>Untitled Page</title>
    <asp:ContentPlaceHolder ID="head" runat="server">
    </asp:ContentPlaceHolder>
 
</head>
<body>
    <form id="form1" ...