pg_trgm: unicode string not working

I am using pg_trgm for spelling correction as prescribed in the
documentation. But I see that it does not work for unicode sring. The
database was initialized with utf8 encoding and the C locale.

Here is the table:
\d words
Table "public.words"
Column | Type | Modifiers
--------+---------+-----------
word | text |
ndoc | integer |
nentry | integer |
Indexes:
"words_idx" gin (word gin_trgm_ops)

Query: select word from words where word % 'कतद';

I get an error:

ERROR: GIN indexes do not support whole-index scans

Any idea what is wrong?

-Sushant.