SQL & PL/SQL

Take the following tables:

--------------------------------------------------------
--  DDL for Table TEST_ASSOCIATION
--------------------------------------------------------

  CREATE TABLE "TEST_ASSOCIATION" 
   (	"PARENT_ID" VARCHAR2(20 BYTE), 
	"CHILD_ID" VARCHAR2(20 BYTE)
   ) ;
--------------------------------------------------------
--  DDL for Table TEST_OBJECT
--------------------------------------------------------

  CREATE TABLE "TEST_OBJECT" 
   (	"ID" NUMBER, 
	"NAME" VARCHAR2(20 BYTE)
   ) ;

Now this query works perfectly:

select * from test_object t_o
where convert(t_o.name, 'US7ASCII') like '%a%'

And this one doesn't (error ORA-01482):

select * from test_object t_o
where
exists (
  select * from test_association t_a
  inner join test_object t_o2
  on t_a.parent_id = t_o2.id
  where t_o2.id = t_o.id
)
and convert(t_o.name, 'US7ASCII') like '%a%'

I suspect the inner query corrupts the CONVERT function, making it fail.

As anyone encountered something like that? Is it possible to correct this?