Unable to drop tablespace

Trying to drop tablespaces in my lab part of my cleanup task. Somehow I can't drop this SIE. Apparently, the materialized log still tied up with the user. Since, I no longer need this tablespace, I just drop the user as well and that solved my problem.

SQL> drop tablespace SIE including contents and datafiles;
drop tablespace SIE including contents and datafiles
*
ERROR at line 1:
ORA-23515: materialized views and/or their indices exist in the tablespace

Note used the same name of the username and the tablespace for better keeping tracking purposes.

SQL> drop user SIE cascade;

User dropped.

SQL> drop tablespace SIE including contents and datafiles;

Tablespace dropped.

SQL>