Still no luck :

SQL>declare
2 cursor c1 is select table_name from dba_tables where owner='TROPP' ;
3 tname varchar2(256):= null ;
4 begin
5 for i in c1
6 loop
7 tname:= tname||i.table_name||',';
8 end loop;
9 dbms_output.put_line(tname);
10 end;
11 /
declare
*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error
ORA-06512: at line 7