Null Reference for PopupControlExtender

Hello,

I am attempting to a web page as a popup via Ajax PopupControlExtender but having some difficulty. When attempting to show the popup, the following error is returned:

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

I've tried to access the control by way of the BehaviorID, ClientID and ControlBehavior (I.e. AjaxControlToolkit.PopupControlBehavior.__VisiblePopup.showPopup(); )

but to no avail. There are a few odd things occurring that maybe worth mentioning:

1.  I am able to show a non-modal popup with Ajax PopupControlExtender in other pages using the BehaviorID

2. I am able to show a Modal popup (I.e. ModalPopupExtender) from the page in question with no problem.

3. Other controls in the page can be accessed with ClientID. It's just the PopupControlExtender control whereby the null reference is returned.

Here's a sample of the code:

<cc1:PopupControlExtender ID="test1_PopupControlExtender" 
        runat="server" DynamicServicePath="" Enabled="True" ExtenderControlID="" 
        TargetControlID="Label1" PopupControlID="test1"></cc1:PopupControlExtender>$find('test1').showPopup();
    //AjaxControlToolkit.PopupControlBehavior.__VisiblePopup.showPopup();
    //$find('<%=form1.FindControl("test1_PopupControlExtender").ClientID%>').showPopup();   

Any help is appreciated.