Could not find control with id. 'btnGuardar' trigger for UpdatePanel 'UpdatePanel1'

I'm getting this error when I use an update panel, and a trigger AjaxControlToolkit accordion control.
<%@ Page Language="vb" AutoEventWireup="false" CodeBehind="frmUpload.aspx.vb" Inherits="DemoFileUploadVB.frmUpload" MasterPageFile="~/BaseMaster.Master" %> Go to the complete details ...