Full Text Index error

SQL> create table t ( a nvarchar2(2000));

Table created.

SQL>
SQL>
SQL> create index idx on t(a) indextype is ctxsys.context;
create index idx on t(a) indextype is ctxsys.context
*
ERROR at line 1:
ORA-29855: error occurred in the execution of ODCIINDEXCREATE routine
ORA-20000: Oracle Text error:
DRG-10509: invalid text column: A
ORA-06512: at "CTXSYS.DRUE", line 160
ORA-06512: at "CTXSYS.TEXTINDEXMETHODS", line 364


solution
Change datatype of column to 'blob' or 'clob'
then try again