Forcing Methuselah to play cards you cannot play

players in table: A B C
player B have 3 vampires (e f g-only g is locked)
player A play enter combat with locked minion and chose e (unlocked vampire),then we see how is target not legal.
what now,roll back,or player A must chose locked target-g vampire?

in a tournament i will sugest to choose a valid target, in a casual play or if he don,t have valid targets ( vamp g in the exemple) he take back his card, put back the replacement card he have draw and shuffle library

DavidR wrote:

in a tournament i will sugest to choose a valid target,

yea,i think same,but one very respectable judge,in online event,give this player roll back,cancel draw, shufle,and this player can take any other action. i want to hear other opinions here.
and beter we talk only on tournaments rules,not frendly game.