Javascript runtime error: Sys is undefined

Hi.

I am using VS 2010.

I am using script manager and update panel in my aspx page. I am getting timeout error. I tried using the below code:

<script type="text/javascript"> 
        Sys.WebForms.PageRequestManager.getInstance().add_endRequest(function (sender, args) { 
            if (args.get_error() && args.get_error().name === 'Sys.WebForms.PageRequestManagerTimeoutException') { 
                            args.set_errorHandled(true); 
            } 
        }); </script> 

How to fix this?

Thanks