UNIX_TIMESTAMP(CURRENT_TIME()) return null ?!?

Hello,
It seems that UNIX_TIMESTAMP(CURRENT_TIME()) returns null, but not UNIX_TIMESTAMP(CURRENT_TIMESTAMP())
Is it wanted?
Or is it a bug?
Thank you for your reply

– --------------------------------------
SELECT
UNIX_TIMESTAMP(CURRENT_TIME()),
UNIX_TIMESTAMP(CURRENT_TIMESTAMP()),
UNIX_TIMESTAMP(NOW())
– --------------------------------------
;