我有这个触发器的错误

create or replace trigger eliminar_serie
before delete on series
for each row
declare
xfecha date;
begin
  select fechaVencimiento into xfecha from series x join licencias y on x.licencia=y.idlicencia;
  if(trunc(xfecha)-trunc(sysdate)>0)then
    raise_application_error(-20004,'No se puede eliminar la serie si no ha vencido la licencia de esta');
  end if;
end;

错误是

Error SQL: ORA-04091: la tabla BD2090938.SERIES está mutando, puede que el disparador/la función no puedan verla
ORA-06512: en "BD2090938.ELIMINAR_SERIE", línea 4
ORA-04088: error durante la ejecución del disparador 'BD2090938.ELIMINAR_SERIE'
04091. 00000 -  "table %s.%s is mutating, trigger/function may not see it"
*Cause:    A trigger (or a user defined plsql function that is referenced in
           this statement) attempted to look at (or modify) a table that was
           in the middle of being modified by the statement which fired it.
*Action:   Rewrite the trigger (or function) so it does not read that table.
sql oracle