LoS lost when summoning a creature

[harmor]

Caster starts casting Summon Monster intending to cast it in the room with an enemy.

Door closes to the room just before the spell finishes.

What happens next?

[Tom Baumbach]

Determinations (like target, or center of radius, etc.) are made when the spell is finished casting. The spell goes off normally, but the caster's options on where the summoned creature can appear are more limited (can't appear inside the room).