Kolom dubbelzinnige fout in Laravel 4

Je kunt dit proberen:

$unit = Unit::join('bookings', 'bookings.unit', '=', 'units.unit')
            ->where('bookings.id', $id)
            ->select(
                'units.id',
                'units.unit',
                'bookings.id as bid',
                'bookings.unit as bunit'
            )
            ->lists('units.unit', 'units.id');