Rare Index with Double Quotes

Came across this rare index.
SQL> select index_name from dba_indexes where tablespace_name='MEDIUM_DATA';

INDEX_NAME

UK_F_PLAN_FC_ACT_jan

F_PLAN_FC_ACT_OLTAS_INDEX1_jan

2 rows selected.

SQL> alter index star.UK_F_PLAN_FC_ACT_jan rebuild;

alter index star.UK_F_PLAN_FC_ACT_jan rebuild

*

ERROR at line 1:

ORA-01418: specified index does not exist
SQL> alter index star."UK_F_PLAN_FC_ACT_jan" rebuild;

Index altered.
SQL>
Found a hint that it needs to be invoked with double quotes ""