wordpress add_cap not working

I'm trying to clone a user role in WordPress to give an editor role slightly more ability.

The following line causes the white screen of death:

$role_owner->add_cap('edit_theme_options');

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Can anyone help to add this simple additional capability?
Full function listed below for reference.

add_action('init', 'createUserRole');

function createUserRole()
{
 global $wp_roles;

 if (!isset($wp_roles))
  $wp_roles = new WP_Roles();

 $role_editor = $wp_roles->get_role('editor');
 $role_owner  = $wp_roles->add_role('owner', 'Site Owner', $role_editor->capabilities);
 $role_owner->add_cap('edit_theme_options');
}

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