RegisterStartupScript not firing off JScript method

I can't seem to figure out how to fire off a javascript function from my code-behind.

Here is my javascript in my aspx page...

<script type="text/javascript">
...
function HidePopup() {
  $(function() {
    $("#popup").dialog("close");
  });
});

I have a similar function called ShowPopup that is fired for the OnClientClick event of a LinkButton. That IS working. However, at the end of a function that is kicked off by pressing this button, I want to hide the JQuery dialog. I have tried the following methods...

ScriptManager.RegisterStartupScript(Me, Me.GetType, "HidePopup", "HidePopup();", True)

and

Page.ClientScript.RegisterStartupScript(Me.GetType(), "HidePopup", "HidePopup();")

I assume I need the first one (I'm making use of the AjaxControlToolkit and this particular button and <div> is in an update panel) but neither one seems to be hiding the dialog box. I get no errors, so I'm kind of lost here. Any ideas? Need more information? Thanks in advance.

EDIT: So my issue isn't really that the javascript isn't firing off, I put in an alert(message) in my HidePopup() function that is going off, so my issue really just seems to be with this line...

$("#popup").dialog("close");

My div is defined as...

<div id="popup" style="display:none;"></div>

Does anybody see anything wrong with that?

Much thanks to you guys, I figured out the issue. In case anyone else stumbles around here...

Since I'm using UpdatePanel I had to use ScriptManager.RegisterStartupScript(<UpdatePanelID>, <UpdatePanelID>.GetType(), "HidePopup", "HidePopup();") - Make note of @VeselinVasilev's answer to use the name of the UpdatePanel in the function call.

Most importantly though, I had to modify my method that actually started the popup. It seems the DOM was losing the reference to the Dialog. I created a global variable and assigned it to the Dialog when it was created. I could then use that to show / hide the Dialog.

var popupDialog;

function ShowPopup(message) {
    $("#popup").html(message);
    popupDialog = $("#popup").dialog({
                    title: "Working",
                    modal: true,
                    dialogClass: "no-close"
    });
};
function HidePopup() {
    popupDialog.dialog("close");
};

If anyone sees a problem with this or has anymore questions definitely let me know.