Calendar Control - Giving Null Object

iam using calender control in asp.net 2.0 as it is using master pages.. iam opening a window form content pages.. on window.close it is giving me null object as its not getting form name code on content page:

<a onclick="openwindow();" ahref="#">
<img src="Images/calendar.gif" style="width: 20px; height: 20px" border=0/></a>
<input ID="Text1" runat="server" readonly="readOnly" type="text" />
function openwindow() {
window.open('/Calender.aspx', 'calendar_window','width=154,height=188');
}
code on opened window from content page
protected void Calender1_SelectionChanged(object sender, EventArgs e)
{
string strjscript = "<script language='javascript'>";
strjscript += "window.opener." + HttpContext.Current.Request.QueryString["formname"];
strjscript += ".value = '" + Calender1.SelectedDate.ToString("yyyy-MM-dd") + "'; window.close();";
strjscript += "</script" + ">";
Literal1.Text = strjscript;
}
protected void Calendar1_dayrender(object sender, DayRenderEventArgs e)
{
if(e.Day.Date==DateTime.Now)
{
e.Cell.BackColor = System.Drawing.Color.LightGray;
}
}