Dark Passenger consistency

The Dark Passenger trait appears to cost mana- but only whenever there is mana. When the creature has no mana (or less than the cost of necrosis), the spell can still be cast.
It would seem (at least to me) that this is inconsistent- it should either not cost mana at all (like most traits that cast based on traits) or, if it costs mana, not cast when there isn’t enough in the creature’s mana pool.