SQL> create table t (col integer);

Table created.

SQL> create or replace trigger trg before insert on t for each row
  2  begin
  3    dbms_output.put_line('in trigger');
  4  end;
  5  /

Trigger created.

SQL> insert into t values(0);
in trigger

1 row created.

SQL> insert into t values ('a');
insert into t values ('a')
                      *
ERROR at line 1:
ORA-01722: invalid number