how to convert nvarchar2 datatype to nclob

Hi all,
How to alter nvarchar2(50) datatype to nclob ........................i will try but it's giving error.

:)
SQL> create table g1(column001 nvarchar2(50));

table created.

sql> alter table g1 modify column001 nclob;
alter table g1 modify column001 nclob
*
ERROR at line 1:
ORA-22858: invalid alteration of datatype

so please solve this ................

Thanks,
Raavi:)