fatal error caused by _bootstrap_book_fix_theme_hooks

I'm getting a fatal error that brings my site completely down for all book pages:

I've narrowed it down to _bootstrap_book_fix_theme_hooks -- if I simply return before it does anything, the error goes away and the navigation is shown albeit completely different.

It doesn't happen on a vanilla simplytest.me site and changing my site (entity, entity_view_mode, features, etc etc) to bartik also works fine.

I'm happy to work on a patch, but I'm not sure what that function is doing or why it's doing it so I was hoping maybe another pair of eyes might be able to give me a nudge in the right direction.

For ease of reference, here's the function in its entirety:

/**
 * Helper function to fix theme hooks in book TOC menus.
 *
 * @param int $bid
 *   The book identification number.
 * @param array $element
 *   The element to iterate over, passed by reference.
 * @param int $level
 *   Used internally to determine the current level of the menu.
 */
function _bootstrap_book_fix_theme_hooks($bid, array &$element, $level = 0) {
  $hook = $level === 0 ? $bid : 'sub_menu__' . $bid;
  $element['#theme_wrappers'] = array('menu_tree__book_toc__' . $hook);
  foreach (element_children($element) as $child) {
    $element[$child]['#theme'] = 'menu_link__book_toc__' . $hook;
    // Iterate through all child menu items as well.
    if (!empty($element[$child]['#below'])) {
      _bootstrap_book_fix_theme_hooks($bid, $element[$child]['#below'], ($level + 1));
    }
  }
}