Web Forms :: How To Close Popup Window Without Alert Box Message

I produce a popup window by adding a hyperlink to a table cell dynamically in my vb.net code behind. Here is the code:

Dim sHyperlink As
String =
"<a href=""LabPopup.aspx?RefNo=" & iLabRefNo &
""" target=""_blank"">" & strValue &
"</a>" objCellValue.Controls.Add(New LiteralControl(sHyperlink))

So on my web page, I will have a list of these hyperlinks and each have a different iLabRefNo. When I click on one of these links, the popup window does appear correctly.

I have a button on the popup window that closes the popup. But I get this message in an alert box when click on my button:

The webpage you are viewing is trying to close the window. Do you want to close the window?

If I click OK then the window close. But I just want to close the popup wondow without this message. What do I need to do?

Here is my code for the button to close the popup:

<asp:Button
ID="cmdLabPopupClose"
runat="server"
Text="Close"
OnClientClick="CloseWindow()"
/>
<script type="text/javascript">
function CloseWindow() {
window.close();
}
</script>