Oracle Database Discussions

in an 11.2.0.3 oracle database, which set undo_management = AUTO, I've done:

• create a new undo tablespace
• set undo_tablespace to point new undo tablespace
• wait for all segments in old undo tablespace seem offline in DBA_ROLLBACK_SEGS
• turn offline old undo tablespace
• and these errors came up:

          ORA-00376: file 111 cannot be read at this time

          ORA-01110: data file 111: '+FRA/orcl/undotbs01.dbf'

I've read it's ok to drop undo tablespace after all segments in undo tablespace turn offline. Why it tried to read old undo tablespace?

Thanks...