confusing error message

I'm having trouble parsing this:

ERROR: aggregate functions are not allowed in FROM clause of their own query level

The example in the regression tests is:

-- LATERAL can be used to put an aggregate into the FROM clause of its query
select 1 from tenk1 a, lateral (select max(a.unique1) from int4_tbl b) ss;
ERROR: aggregate functions are not allowed in FROM clause of their own query level
LINE 1: select 1 from tenk1 a, lateral (select max(a.unique1) from i...
^
I think the "own query level" of the max aggregate function in this case
is the subquery "ss", and so it's not in the FROM clause of its own
query level.

It's understandable why this is not allowed, but I don't think the error
message explains it. Could we come up with a better wording?