convert with libpq

Hi

I must convert some records from database. 
I have table named "langset" in unicode database. When I try use command 

select convert((SELECT "1" from langset where id_langset=5) using 
utf_8_to_iso_8859_2)

in PGadmin, all is ok, sql return 1 record. But where i execute the same 
command in libpq I got 0 rows:
res=PQexec(conn_lokalne,"select convert((SELECT \"1\" from langset where 
id_langset=5) using utf_8_to_iso_8859_2)");

printf("%s",PQgetvalue(res,0,0));

What is the problem?