PopupControlExtender - this._postbackSettings.async is null or not an object

Hello.

I am having some trouble with the PopupControlExtender. I have a Panel (panel 1) that contains some text and a button (button 1). I have another button (button 2) that I want to cause the panel to popup. I set the popupControlExtender TargetControlID as button 2 and the PopupControlID as the Panel.

When I click button 2 the panel appears as expected but if I now click button 1 that is on the panel (i.e. causing a postback) I get a "this._postbackSettings.async is null or not an object"javascript error.

Does anyone have any ideas that can help?

Thanks!

Ian