[Question] Problem with this code I'm using to change abilities.

[Soda26]

I found this code that let's me change a pokemon's ability. The only problem is that when the player presses B it crashes the game. (other than that the code works great!
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def pbChangeChoose
pbChoosePokemon(1,3)
pkmn=$Trainer.party[$game_variables[1]] if $game_variables[1] >= 0
if pkmn
options = pkmn.getAbilityList.map { |ability, _| GameData::Ability.get(ability).name }
choice = pbMessage("What ability to you want?",options,options[pkmn.ability_index])
pkmn.ability_index = choice
end
end
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The Error

comparison of String with 0 failed
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I tried adding a branch in the case where choice = 0 or nil, but it didn't work.