ORA-01502 and ORA-00604 when dropping a user

SQL> drop user test cascade;
drop user test cascade*ERROR at line 1:ORA-00604: error occurred at recursive SQL level 2ORA-01502: index 'SYS.I_SEQ1' or partition of such index is in unusable state
connect as sys
SQL> alter index SYS.I_SEQ1 rebuild online;
Index altered.

SQL> drop user test cascade;
User dropped.