Issue with json_agg() and ordering

Hi,

Could anyone give me a few pointers as to how I might resolve the following :

select json_agg(my_table) from (my_table) where foo='test' and bar='f'
order by last_name asc, first_name asc;

ERROR: column "my_table.last_name" must appear in the GROUP BY clause
or be used in an aggregate function
LINE 1: ...foo='f' order by last_name ...

I suspect It doesn't really matter what my table looks like for the
purposes of the above, but if you need something to go by:

create table my_table(
last_name text,
first name text,
foo text,
bar boolean
);

Thanks !

Bob