Redirect to new page with multiple Parameters using JavaScript Window.open in ASP.Net

Hi

how to redirect to new page in Window.open

In Window.open second page is not redirecting in multiple Parameter. I tried in all ways.

Still not Opening. 

Output Link Coming like this

http://localhost:63598/CrmProjectFinal1_current/Rpt_Preview.aspx?pamendtype=%22%20+%20custtype%20+%20%22&pamendid=%22%20+%20custid%20+%20%22 

<input type="button" value="Link html butt" OnClick="ChkPrevious()"/>

<script type='text/javascript'>
    function ChkPrevious() {
        var custtype = document.getElementById("<%= lblcustType.ClientID %>").value;
        var custid = document.getElementById("<%= lblcustID.ClientID %>").value;
        window.open('Rpt_Preview.aspx?pamendtype=" + custtype + "&pamendid=" + custid + "', '_blank');
    }
</script>