ToggleButtonExtender doesn't reflect Checked=true in code

I have a CheckBox with a ToggleButtonExtender to make it look nice.  If I click the image, it toggles and the appropriate image displays.  When a set the Checkbox1.Checked=true in code, the checkbox is toggled, but the image of the ToggleButtonExtender doesn't change.

How do you force the visual change when setting CheckBox1.Checked=true in code?