digest as IMMUTABLE function

Hello,

We were trying to create an index using the digest function like that (where
picture is a bytea field):

create unique index my_index on my_table digest(picture, 'md5'));

but we got the error:

ERROR: functions in index expression must be marked IMMUTABLE

So, we have created the function digestinmutable(), like that:

CREATE OR REPLACE FUNCTION digestinmutable(bytea, text)
RETURNS bytea AS
'$libdir/pgcrypto', 'pg_digest'
LANGUAGE 'c' IMMUTABLE;

And after that, we have created the index using the new function
(digestinmutable) :

create unique index my_index on my_table digestinmutable(picture, 'md5'));

IT SEEMS TO WORK OK. Does Anybody know if we'll have any problem using md5
in that way?

Thank you very much for your help.

Bye