XPages checkboxgroup styling

The following is my code for a basic checkbox group in XPages:

<div class="checkbox">
    <xp:checkBoxGroup id="checkBoxGroup1">
        <xp:selectItem itemLabel="First" itemValue="1"></xp:selectItem>
        <xp:selectItem itemLabel="Second" itemValue="2"></xp:selectItem>
        <xp:selectItem itemLabel="Third" itemValue="3"></xp:selectItem>
    </xp:checkBoxGroup>
</div>

When I preview the XPage, I only see the labels. The checkbox is missing. If I remove the div with the Bootstrap class (checkbox), the checkboxes appear.

Would someone clue me in to what I should do if I want to add the Bootstrap class to the checkbox group?

Thanks,

Dan