ERROR 1111 (HY000). what's wrong? incorrect script of the function? HELP! [требует правки]

mysql> create function infoReader(readerName char(30)) RETURNS CHAR (100) begin
-> DECLARE maxID INT;
-> declare maxexpiry date;
-> SELECT issuing.id_issuing into maxID FROM Issuing WHERE issuing.issuing_date = max(issuing.issuing_date) and issuing.reader = (select reader.ticket from reader where reader.FIO =readerName);
-> IF (maxexpiry>CURRENT_DATE) then return ('закрыто'); else return (select concat(book.ID_Book, " "", book.name, ""  ")from book, content_issuing where book.ID_Book = content_issuing.book and content_issuing.issuing = maxID);
-> end if;  end;$$

Query OK, 0 rows affected (0.06 sec)
mysql> set @readerName = 'Кочеткова Д.С.';`
-> $$`
Query OK, 0 rows affected (0.02 sec)
mysql> delimiter ;
mysql> select infoReader(@readerName);`
ERROR 1111 (HY000): Invalid use of group function