19.0.0.0.0 : ORA-01031 "insufficient privileges"

Hi,

I am having a strange problem :

sqlplus / as sysdba ;

SQL> create table USER1.toto2 as select * from dba_objects where 1=2;

Table USER1.TOTO2 created.

SQL> ALTER TABLE USER1.toto2 ADD (id2 number);

Table USER1.TOTO2 modified.

SQL> ALTER TABLE USER1.toto2 MODIFY (id2 VARCHAR2(3));

ORA-01031: privilèges insuffisants

01031. 00000 -  "insufficient privileges"

*Cause:    An attempt was made to perform a database operation without

           the necessary privileges.

*Action:   Ask your database administrator or designated security

           administrator to grant you the necessary privileges

Why the "modify column" does not work when the "add column" works well !!!

Thanks.

GD.