Ajax Calendar Extender causes "0x800a01bd - JavaScript runtime error: Object doesn't support this action"

[JimInAz]

Working on a simple ASP.net webforms project and have a page with a standard text box and a calendar extender from AJAX v16.1. When I run the project I get "0x800a01bd - JavaScript runtime error: Object doesn't support this action" on the calendar extender. Have found NOTHING useful on the web about this. Any clue what's happening or what I can do to fix it?
-Yes, Ajax is installed correctly. I have Ajax combo boxes on the form and they're not causing any problems.

<asp:TextBox ID="txtQuestion_4" runat="server" Width="85px">/asp:TextBox
<asp:CalendarExtender ID="txtQuestion_4_CalendarExtender" runat="server" BehaviorID="txtQuestion_4_CalendarExtender" TargetControlID="txtQuestion_4">/asp:CalendarExtender

[alekitto]

Page markup and/or stack trace could be useful :)
What browser are you using for displaying the page?

Anyway, i've never worked in ASP.NET, i'm not sure how to replicate this...