Target Retainer Cancelled

Situation: Vampire A and vampire B are in combat. Vampire A has a retainer. Vampire B chooses a handstrike and plays Target Retainer on that strike, targeting A's retainer. Target Retainer is cancelled (I don't think it matters if it's cancelled due to its own cardtext or due to some other cancellation effect).

What happens to B's strike? Does it default back to targeting A, or fizzle, or something else?

Name: Target Retainer
Cardtype: Combat
Aim. Only usable as this minion chooses a strike. A minion can play only one aim each strike.
You can target a retainer on the opposing minion with this strike (instead of the opposing minion). Or you can target the opposing minion directly as usual, but if any damage from this strike is successfully inflicted, this strike inflicts +1 damage. The opposing minion can discard two combat cards{} to cancel this card as it is played.