[Error] [17.2] Mega Evolution Error

[Pe_Zero]

Hi there, I've wanted to add Mega Evolution to my game for some time. However, whenever I try giving my Pokemon the Stone while having the Key Stone myself and enter a battle, I select the 'Battle' option, but always get the same message:

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Pokemon Essentials
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[Pokémon Essentials version 17.2]

Exception: NoMethodError

Message: undefined method `each' for :MEGARING:Symbol

PokeBattle_Battle:543:in `pbHasMegaRing?'

PokeBattle_Battle:1687:in `pbCanMegaEvolve?'

EliteBattle_UI:1122:in `pbFightMenu'

PokeBattle_Battle:2612:in `pbCommandPhase_ebs'

PokeBattle_Battle:2606:in `loop'

PokeBattle_Battle:2696:in `pbCommandPhase_ebs'

PokeBattle_Battle:2595:in `each'

PokeBattle_Battle:2595:in `pbCommandPhase_ebs'

EliteBattle_Battle:294:in `pbCommandPhase'

EliteBattle_Battle:224:in `pbStartBattleCore'

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OK   
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I tried looking into the code, but there doesn't seem to be a single thing wrong with it. Also, what's worth noting is that the enemy trainers can freely mega evolve their Pokemon when I give them the Mega Stone, so it has to do something with the Keystone, however, I have no idea what exactly.