[GENERAL] can't cast hstore to json

[Joe Van Dyk]

I've got a postgresql 9.3.2 server, compiled from scratch. Getting this
error:

# select hstore('a', 'b')::json;
ERROR:  cannot cast type hstore to json
LINE 1: select hstore('a', 'b')::json;
                               ^
# select array_to_json(array[hstore('a', 'b')]);
  array_to_json
------------------
 ["\"a\"=>\"b\""]
(1 row)

(I expected  [{"a": "b"}] )

I don't get this on other postgresql installations. Any ideas?

Joe