Why won't MEDIAN() work?

It doesn't like MEDIAN(user_registration) when it's left as a BINARY(14); that's reasonable.  It still doesn't like it if I cast it; that isn't.

Answer: cast or do some other operation on the *result* of MEDIAN().  That's kind of nutty.

SQL

WITH sub(user_id, user_registration) AS
(
  SELECT user_id, CAST(user_registration AS DOUBLE)
  FROM user
  LIMIT 10
)
SELECT *, MEDIAN(user_id) OVER() AS `MEDIAN(user_id) works` FROM sub;

WITH sub(user_id, user_registration) AS
(
  SELECT user_id, CAST(user_registration AS DOUBLE)
  FROM user
  LIMIT 10
)
SELECT *, MEDIAN(user_registration) OVER() AS `MEDIAN(user_registration) doesn't` FROM sub;

WITH sub(user_id, unixtime) AS
(
  SELECT user_id, UNIX_TIMESTAMP(user_registration)
  FROM user
  LIMIT 10
)
SELECT *, MEDIAN(unixtime) OVER() AS `Hey, unix_timestamp() is supposed to return an unsigned int!` FROM sub;

WITH sub(user_id, unixtime) AS
(
  SELECT user_id, UNIX_TIMESTAMP(user_registration) / 10
  FROM user
  LIMIT 10
)
SELECT *, MEDIAN(unixtime) OVER() AS `Maybe the numbers are just too big?` FROM sub;

WITH sub(user_id, unixtime) AS
(
  SELECT user_id, UNIX_TIMESTAMP(user_registration)
  FROM user
  LIMIT 10
)
SELECT *, CAST(MEDIAN(unixtime) OVER() AS DOUBLE) AS `Resultset 3 again, casting the *result*` FROM sub;

SQL

Checking query status...