ORA-01858 while inserting into an immutable table

I'm trying the quite new immutable tables features and run into an unexpected ORA-01858 error.

Oracle database 19.21 (standard 2 / on-premise) running on a RedHat 8.8

The sql statements are :

create immutable table test_imu 

(

test NUMBER(1) DEFAULT 0 NOT NULL

) 

NO DROP UNTIL 1 DAYS IDLE   

NO DELETE UNTIL 16 DAYS AFTER INSERT

ENABLE ROW MOVEMENT;

insert into test_imu (test) values (1);

Error report -

SQL Error: ORA-01858: a non-numeric character was found where a numeric was expected

01858. 00000 - "a non-numeric character was found where a numeric was expected"

*Cause:  The input data to be converted using a date format model was

      incorrect. The input data did not contain a number where a number was