diff --git a/packages/backend/src/core/CustomEmojiService.ts b/packages/backend/src/core/CustomEmojiService.ts
index dc365986fe..3de936dd65 100644
--- a/packages/backend/src/core/CustomEmojiService.ts
+++ b/packages/backend/src/core/CustomEmojiService.ts
@@ -43,12 +43,8 @@ export class CustomEmojiService {
 			lifetime: 1000 * 60 * 30, // 30m
 			memoryCacheLifetime: 1000 * 60 * 3, // 3m
 			fetcher: () => this.emojisRepository.find({ where: { host: IsNull() } }).then(emojis => new Map(emojis.map(emoji => [emoji.name, emoji]))),
-			toRedisConverter: (value) => JSON.stringify(value.values()),
-			fromRedisConverter: (value) => {
-				// 原因不明だが配列以外が入ってくることがあるため
-				if (!Array.isArray(JSON.parse(value))) return undefined;
-				return new Map(JSON.parse(value).map((x: Emoji) => [x.name, x]));
-			}, // TODO: Date型の変換
+			toRedisConverter: (value) => JSON.stringify(Array.from(value.values())),
+			fromRedisConverter: (value) => new Map(JSON.parse(value).map((x: Emoji) => [x.name, x])), // TODO: Date型の変換
 		});
 	}
 
diff --git a/packages/backend/src/misc/cache.ts b/packages/backend/src/misc/cache.ts
index a4abd4f878..d35414acf7 100644
--- a/packages/backend/src/misc/cache.ts
+++ b/packages/backend/src/misc/cache.ts
@@ -8,7 +8,7 @@ export class RedisKVCache<T> {
 	private memoryCache: MemoryKVCache<T>;
 	private fetcher: (key: string) => Promise<T>;
 	private toRedisConverter: (value: T) => string;
-	private fromRedisConverter: (value: string) => T | undefined; // undefined means no cache
+	private fromRedisConverter: (value: string) => T;
 
 	constructor(redisClient: RedisKVCache<T>['redisClient'], name: RedisKVCache<T>['name'], opts: {
 		lifetime: RedisKVCache<T>['lifetime'];
@@ -92,7 +92,7 @@ export class RedisSingleCache<T> {
 	private memoryCache: MemorySingleCache<T>;
 	private fetcher: () => Promise<T>;
 	private toRedisConverter: (value: T) => string;
-	private fromRedisConverter: (value: string) => T | undefined; // undefined means no cache
+	private fromRedisConverter: (value: string) => T;
 
 	constructor(redisClient: RedisSingleCache<T>['redisClient'], name: RedisSingleCache<T>['name'], opts: {
 		lifetime: RedisSingleCache<T>['lifetime'];