boom/bust targeting a fetchland

[Sevryn]

boom:
Destroy target land you control and target land you don't control

situation:
turn 3, have 2 mountains and a fetchland.
cast boom targeting my fetchland (and a land of opponent)
in response and without passing priority, use fetchland

does boom 'fizzle'?

[dorsch]

Boom requires two targets to be cast.
A spell is countered on resolution (fizzles) when _all_ of its targets are illegal. Otherwise it will do as much as possible.