Ribbon v3.0 problem with checkboxes and radiobuttons

I have a radio button defined in a ribbon tab:

<ribbon:RadioButton Label="Prepend" Click="OnRadioButtonClick" />

private void OnRadioButtonClick( object sender, RoutedEventArgs e )
{
RibbonControls.RadioButton clickedRadioButton = sender as RibbonControls.RadioButton;
clickedRadioButton.IsChecked = true;
...
...
}

Yet, when I click it, it does not get selected. The simple behavior I'm wanting is I have a group of 3 radio buttons and I want them to act as a radio button group, i.e. only one can be selected at a time.