Testing a Hive array for IS NULL says not null

I have a table containing an array, and I want to check if it is empty or NULL. It appears that I cannot check the NULL directly! Can anyone shed light on why the NULL check isn’t working?

create table `test_array_split`
( `campaign` string
, `questions` array<string> 
) 
stored as orc ;

insert into `test_array_split` (`campaign`) values ('1');

select
  `campaign`
, `questions`
, size(`questions`)
, case when `questions` is null then 'null' else 'not null' end `isnull`
from `test_array_split`;
+-----------+------------+------+-----------+
| campaign  | questions  | _c2  |  isnull   |
+-----------+------------+------+-----------+
| 1         | NULL       | -1   | not null  |
+-----------+------------+------+-----------+