Jquery for checkbox doesn't work for model popup

In ModalPopupExtender control I have a checkbox. On checkbox checked I want the Submit button in the popup to be enabled else disabled.

  src="http://ift.tt/VorFZx"></script>
    <script type = "text/javascript">

function Checked() {
                if(document.getElementById('<%=chkUser.ClientID%>').checked)
                {
                    document.getElementById('<%=btnSubmit.ClientID%>').EnableSubmitButton = true;
                }
            }
    
   </script>

I'm getting the following error. 0x800a1391 - JavaScript runtime error: Checked undefined error.