ORA-06502 PL SQL numeric or value error character string buffer too small

Question

I experimented with the following code in many ways, such as by removing the while or the if, but whenever I combine the two (if and while), I consistently receive the error at the end.

undefine numero
set serveroutput on
accept numero prompt 'Type # between 100 and 999: '
declare
   i number:=1;
   a char(25);
   b char(1);
   c varchar2(10);
   d number;
begin
   c := №
   d := length(c);
   b := substr(c, i, 1);
   while i <= d loop
     if b = '1' then
       a:= a||'one ';
     end if;
     i := i+1;
   end loop;
   dbms_output.put_line('The number is '||a);
end;
/

ERROR:

ORA-06502: PL/SQL: numeric or value error: character string buffer too small
ORA-06512: at line 13
06502. 00000 -  "PL/SQL: numeric or value error%s"

FIXED by changing how I declared the variable "a" to:

a varchar2(2000);

*Notice that here, the significant change is to use VARCHAR2 instead of CHAR (not the bigger length). According to @user272735 's answer, that's the key.