diff --git a/src/field/arena.ts b/src/field/arena.ts
index 484450cc5df..ed2f1df9b12 100644
--- a/src/field/arena.ts
+++ b/src/field/arena.ts
@@ -145,7 +145,7 @@ export class Arena {
             ? BiomePoolTier.BOSS_SUPER_RARE
             : BiomePoolTier.BOSS_ULTRA_RARE;
     console.log(BiomePoolTier[tier]);
-    while (!this.pokemonPool[tier].length) {
+    while (!this.pokemonPool[tier]?.length) {
       console.log(`Downgraded rarity tier from ${BiomePoolTier[tier]} to ${BiomePoolTier[tier - 1]}`);
       tier--;
     }