ToggleButton doesn't toggle when set in code

I have a toggle button that works fine, applied to a CheckBox (ToggleButtonExtender).  When I click it, the images change and everything works fine.  I have a situation where I need to set the Checked property to true in code.  When I do this, the ToggleButton doesn't reflect this change visually even though the CheckBox is "checked".  How do you force this change visually to make it programmatically change its images in accordance with the state of the CheckBox?

Its as if the ToggleButton(Extender) doesn't respond to programmatic changes, only click events.  How do you circumvent this?

Thanks,

-D