diff --git a/code/modules/mob/living/simple_animal/simple_animal.dm b/code/modules/mob/living/simple_animal/simple_animal.dm
index 58147a598f1..d7e94514dc2 100644
--- a/code/modules/mob/living/simple_animal/simple_animal.dm
+++ b/code/modules/mob/living/simple_animal/simple_animal.dm
@@ -638,7 +638,8 @@
 /mob/living/simple_animal/onTransitZ(old_z, new_z)
 	..()
 	if(AIStatus == AI_Z_OFF)
-		SSidlenpcpool.idle_mobs_by_zlevel[old_z] -= src
+		var/list/idle_mobs_on_old_z = LAZYACCESS(SSidlenpcpool.idle_mobs_by_zlevel, old_z)
+		LAZYREMOVE(idle_mobs_on_old_z, src)
 		toggle_ai(initial(AIStatus))
 
 /mob/living/simple_animal/proc/add_collar(obj/item/petcollar/P, mob/user)