Strange Behaviour Related to ORA-01400 Error

Question

I'm creating a table on our database with version Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64b

create table tahakkuk_kalem(
                           id       int generated always as identity primary key,       
                           aciklama varchar2(250),
                           tur      int default 0 not null 
                          );

and then applying some insert statements :

SQL> insert into tahakkuk_kalem(aciklama) select 'Atıksu' from dual;

SQL> select * from tahakkuk_kalem;

ID  ACIKLAMA  TUR
--  --------  --- 
1   Atıksu     0

SQL> rollback;

SQL> insert into tahakkuk_kalem(aciklama,tur) select 'Atıksu',0 from dual;

ID  ACIKLAMA  TUR
--  --------  --- 
2   Atıksu     0

SQL> rollback;

So far so good, but if I issue the statement below ;

SQL> insert into tahakkuk_kalem(aciklama,tur) 
     select 'Atıksu',0 from dual union all select 'Ceza',0 from dual;

met an incomprehensible error statement :

ORA-01400: cannot insert NULL into ("MY_SCHEMA"."TAHAKKUK_KALEM"."ID")

Do you have any opinion what could be the reason ?