NoMethodError when using a Pokemon holding a Z Crystal

[chippy]

Hi! Whenever a turn starts or ends while my Aevium Milotic holding a Waterium-Z is in battle, the game gives the attached error message, skipping the enemy's turn (including sending a Pokemon in after fainting).  Issue is not present with any other Pokemon; I can just switch out my Milotic to continue the battle, but I would like to use Z-moves without breaking the battle system. This issue does not occur when using Erin's Staraptor during the Diamond Route. I am playing on 13.0.5, but the error message says 13.0.4. I tried using a fresh install, but that didn't fix the issue. I hope it's not my save --I usually play with mods, but this issue occurs whether or not mods are loaded.