can't cast hstore to json

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