有大神帮忙看下哪儿写错了

set serveroutput on;
declare
cursor cemp is select to_char(hiredate, 'yyyy') from emp;
phiredate varchar2(4);
count80 number := 0;
count81 number := 0;
count82 number := 0;
count87 number := 0;
begin
open cemp;
loop
fetch cemp into phiredate;
exit when cemp%notfound;
if phiredate='1980' then count80:=count80+1;
elsif phiredate='1981' then count81:=count81+1;
elsif phiredate='1982' then count82:=count82+1;
else count87 := count87 + 1;
end if;
end loop;
close cemp;
dbms_output.put_line('Total '||count80 + count81 + count82 + count87);
dbms_output.put_line('1980 '||count80);
dbms_output.put_line('1981 '||count81);
dbms_output.put_line('1982 '||count82);
dbms_output.put_line('1987 '||count87);
end;
/
错误报告 -
ORA-06502: PL/SQL: 数字或值错误 : 字符到数值的转换错误
ORA-06512: 在 line 20
06502. 00000 - "PL/SQL: numeric or value error%s"
*Cause: An arithmetic, numeric, string, conversion, or constraint error
occurred. For example, this error occurs if an attempt is made to
assign the value NULL to a variable declared NOT NULL, or if an
attempt is made to assign an integer larger than 99 to a variable
declared NUMBER(2).
*Action: Change the data, how it is manipulated, or how it is declared so
that values do not violate constraints.