Using a three state ribbon checkbox in a ribbon popup button

I would like to have a popup button in my ribbon that contains checkboxes with IsThreeState enabled.

I tried adding some simple xaml to one of my views and set IsTogged to null, but the visual for the indeterminate state is not showing up.  It appears as if the value is false.

<actiproRibbon:PopupButton Label="MyPopup">
    <actiproRibbon:Menu>
        <actiproRibbon:CheckBox Label="My checkbox" IsThreeState="True" IsChecked="{Binding IsToggled}" ></controls:CheckBox>
    </actiproRibbons:Menu>
</actiproRibbon:PopupButton>

Is there something else I need to change to show the indeterminate state?

Thanks!