Foreign Keys

I am able to enter a null in a foreign key field in one table(table2)
without the foreign key relationship (table1.t1_id) validating the value.
Why is this?

Below are 2 table's scripts:

create table table1 (
t1_id VARCHAR(32) not null,
t1_desc VARCHAR(32) null,
constraint table1_pkey primary key (t1_id) ) ;

create table table2 (
t2_id VARCHAR(32) not null,
t1_id VARCHAR(32) null,
t2_desc VARCHAR(32) null,
constraint table2_pkey primary key (t2_id) ) ;

create index table2_t1_id
on table2 ( t1_id ) ;

alter table table2
add constraint table1_table2_t1_id_FK1
foreign key ( t1_id )
references table1 ( t1_id )
ON DELETE RESTRICT ON UPDATE CASCADE ;

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