SQL & PL/SQL

Hi,
Oracle give the error ORA-01418 when I try to do the following;

Create unique index t1_pk on TABLE1(EntryId DESC) ;

If the I try to add primary key Contraint using the above index t1_pk as below:
ALTER TABLE TABLE1 ADD CONSTRAINT TABLE1_PK PRIMARY KEY (EntryId) USING INDEX t1_pk;

the following error comes "ORA-01418: specified index does not exist" but the index t1_pk exists.

My requirement is to create a descending index on EntryId which gets applied for all the queries.

Thanks
Tuhin