Operand should contain 1 column

[Isaco Mua]

I have an IF in a select section query that i need help with...
I know this can be done with LEFT JOIN but i need this to work if possible.

this works fine:
SELECT ag . * , IF( ag.is_what = 'table2',( SELECT ac.ID FROM accounting_checks ac WHERE ac.ID=ag.related_ID ) , '' )
FROM table ag
WHERE ag.user_ID =1234

BUT this gives me an error:Operand should contain 1 column

SELECT ag . * , IF( ag.is_what = 'table2',( SELECT ac.* FROM accounting_checks ac WHERE ac.ID=ag.related_ID ) , '' )
FROM table ag
WHERE ag.user_ID =1234