Sorting via relationship where null_last = true gives error

[allyzaconde]

Trying to sort via relationship that is nullable gives this error, and in configs/purity.php null_last = true
making null_last = false removes this error

SQLSTATE[42S22]: Column not found: 1054 Unknown column 'iso_code' in 'order clause' (Connection: mysql, SQL: select * from press_release_address_overviews where is_international = 1 order by iso_code is null, (select countries.iso_code from countries where countries.id = press_release_address_overviews.country_id order by countries.iso_code desc limit 1) desc limit 10 offset 0)