ORA-30025: DROP segment ‘_SYSSMU7_2070203016$’ (in undo tablespace) not allowed

[oracle@x3dbzx36 ~]$ oerr ora 30025
30025, 00000, "DROP segment '%s' (in undo tablespace) not allowed"
// *Cause:   Explicit DROP operation is not allowed on undo tablespace segments
// *Action:  Check the undo segment name and reissue statement if
//           necessary.

kIsH@x3z6zx9<^>alter rollback segment "_SYSSMU7_2070203016$" offline;

Rollback segment altered.

kIsH@x3z6zx9<^>alter system set "_OFFLINE_ROLLBACK_SEGMENTS"="_SYSSMU7_2070203016$" scope=spfile;

System altered.

[oracle@x3dbzx36 ~]$ srvctl stop database -d x3z6zx9
[oracle@x3dbzx36 ~]$ srvctl start database -d x3z6zx9

kIsH@x3z6zx9<^>drop rollback segment "_SYSSMU7_2070203016$";

Rollback segment dropped.