adding supplemental log data not finished

hello. I try to give that command but there is no respond from sqlplus. I use Oracle 12c oda machine.

SQL> alter database add supplemental log data;

In alert log;

Tue Jun 07 22:45:13 2016
SUPLOG: Previous supplemental logging attributes at scn = 206679905442
SUPLOG:  minimal = OFF, primary key = OFF
SUPLOG:  unique = OFF, foreign key = OFF, all column = OFF
SUPLOG:  procedural replication = OFF
Tue Jun 07 22:45:14 2016
SUPLOG: New supplemental logging attributes at scn = 206679905534
SUPLOG:  minimal = ON, primary key = OFF
SUPLOG:  unique = OFF, foreign key = OFF, all column = OFF
SUPLOG:  procedural replication = OFF

there is No "completed" message and my command stil works. When i give break, then I try to drop supplemental log data but there is no respond too. I can drop after "alter system switch logfile". What may the problem be? thanks