How to call javascript function from code behind

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ASP.NET

Javascript

printing

I want to print asp panel but when i call js function from code behind it gives me this error

JavaScript runtime error: Unable to get property 'getElementsByTagName' of undefined or null reference

how to solve this error????



my script is

<script>

function printform()
        {

            var printContent = document.getElementById('<%= PNL_PrxStatus.ClientID %>');
             var btn = printContent.getElementsByTagName("input");

             var windowUrl = 'about:blank';
             var uniqueName = new Date();
             var windowName = 'Print' + uniqueName.getTime();
             var printWindow = window.open(windowUrl, windowName, 'left=0,top=0,width=0,height=0');

             printContent.setAttribute("Position", "absolute");
             printContent.setAttribute("Top", "1291px");
             printContent.setAttribute("Left", "1341px");
             btn[0].style.display = 'none';

             printWindow.document.write(printContent.innerHTML);
             printWindow.document.close();
             printWindow.focus();
             printWindow.print();
             printWindow.close();
     }


</script>

and my code behide is this


ScriptManager.RegisterClientScriptBlock(Me.Page, GetType(String), "PrintForm", "printform();", true)


thnx in advance....