postgresql - How to create index on json field in Postgres 9.3 -

in postgresql 9.3 beta 2 (?), how create index on json field? tried using -> operator used hstore got following error:

 create table publishers(id int, info json);  create index on publishers((info->'name')); 

error: data type json has no default operator class access method "btree" hint: must specify operator class index or define default operator class data type.

found:

create table publishers(id int, info json);  create index on publishers((info->>'name'));