declare
    l_text varchar2(300 char) := 'begin';
begin
  for j in ( select level lvl from dual connect by level < 3 )
  loop
      l_text := l_text || 
                     case
                     --Number
                     when j.lvl = 1 then j.lvl
                     --String
                     when j.lvl = 2 then 'asdasdasdas'
                     end;
  end loop;
end;  
--------------------------------------
ORA-06502: PL/SQL: numeric or value error: character to number conversion error
ORA-06512: at line 11

declare
    l_text varchar2(300 char) := 'begin';
begin
  for j in ( select level lvl from dual connect by level < 3 )
  loop
      l_text := l_text || 
                     case
                     --Number
                     when j.lvl = 1 then to_char(j.lvl)
                     --String
                     when j.lvl = 2 then 'asdasdasdas'
                     end;
  end loop;
end;