undo tablespace problem , urgent plz?

[madhugp]

to reduce the size of my undo tablespace ' UNDOTBS1' I used the following method..

CREATED ANOTHER TABLESPACE UNDOTBS2;

ALTER SYSTEM SET UNDO TABLESPACE = 'UNDOTBS2';
ALTERED SUCCESSFULLY.

NOW I WANT TO DROP THE FIRST TABLESPACE , BUT
WE SHOULD MAKE IT OFFLINE FIRST.

ALTER TABLESPACE UNDOTBS1 OFFLINE;

GIVES ORA-30042 CANNOT MAKE OFFLINE

WHY HAPPEND THIS. PLZ SUGGEST?