can't cast hstore to json

[Joe Van Dyk <joe@xxxxxxxxx>]

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