[Question] Ability-Swapping NPC doesn't function? What am I doing wrong?

I've added a NPC to Oak's Lab who is supposed to change the ability of the Pokemon in Slot 1 when asked. But he doesn't seem to be able to change the abilities of any of my Pokemon. Even when I try using a Pokemon with two different regular abilities and a different hidden ability.

Here's his code(with all the in-character "Hi there!" talking removed):

Select an Ability for your Pokemon in Slot 1.
\ch[1,-1,
Reset Ability,Ability 1,Ability 2,Ability 3,Exit]

Then I've got the "Conditional branch variable, temp pokemon choice == 0" stuff, the same for choices 1, 2, 3, and Exit(which just jumps to the label above the "Okay, bye!" text at the end of his event).

Reset ability does this:
pkmn = $Trainer.first_able_pokemon
pkmn.ability_index = nil

Ability 1 does this:
pkmn = $Trainer.first_able_pokemon
pkmn.ability_index = 0

Ability 2 does this:
pkmn = $Trainer.first_able_pokemon
pkmn.ability_index = 1

Ability 3 does this:
pkmn = $Trainer.first_able_pokemon
pkmn.ability_index = 2

This code seems simple and it should work, so what's going wrong? Why is it that when I talk to him and select an option, nothing happens to the ability of my Pokemon in slot 1?

Turns out the trick is to use this:

pkmn = $Trainer.party[0]

Not this:
pkmn = $Trainer.first_able_pokemon

I have no idea why pkmn = $Trainer.first_able_pokemon worked for EV/IV and name editing but not for Ability editing.