diff --git a/packages/backend/src/misc/cache.ts b/packages/backend/src/misc/cache.ts
index e472acd38..ee6e90a66 100644
--- a/packages/backend/src/misc/cache.ts
+++ b/packages/backend/src/misc/cache.ts
@@ -1,7 +1,7 @@
 export class Cache<T> {
-	public cache: Map<string | null, { date: number; value: T; }>;
+	public cache: Map<string, { date: number; value: T; }>;
 	private lifetime: number;
-	public fetcher: (key: string | null) => Promise<T | undefined>;
+	public fetcher: (key: string) => Promise<T | undefined>;
 
 	constructor(lifetime: number, fetcher: Cache<T>['fetcher']) {
 		this.cache = new Map();
@@ -9,14 +9,14 @@ export class Cache<T> {
 		this.fetcher = fetcher;
 	}
 
-	public set(key: string | null, value: T): void {
+	public set(key: string, value: T): void {
 		this.cache.set(key, {
 			date: Date.now(),
 			value,
 		});
 	}
 
-	public get(key: string | null): T | undefined {
+	public get(key: string): T | undefined {
 		const cached = this.cache.get(key);
 		if (cached == null) return undefined;
 
@@ -29,7 +29,7 @@ export class Cache<T> {
 		return cached.value;
 	}
 
-	public delete(key: string | null): void {
+	public delete(key: string): void {
 		this.cache.delete(key);
 	}
 
@@ -38,7 +38,7 @@ export class Cache<T> {
 	 * run to get the value. If the fetcher returns undefined, it is
 	 * returned but not cached.
 	 */
-	public async fetch(key: string | null): Promise<T | undefined> {
+	public async fetch(key: string): Promise<T | undefined> {
 		const cached = this.get(key);
 		if (cached !== undefined) {
 			return cached;
diff --git a/packages/backend/src/services/note/create.ts b/packages/backend/src/services/note/create.ts
index e7dac0886..77a6e4c47 100644
--- a/packages/backend/src/services/note/create.ts
+++ b/packages/backend/src/services/note/create.ts
@@ -266,7 +266,7 @@ export default async (user: { id: User['id']; username: User['username']; host:
 	incNotesCountOfUser(user);
 
 	// Word mute
-	mutedWordsCache.fetch(null).then(us => {
+	mutedWordsCache.fetch('').then(us => {
 		for (const u of us) {
 			checkWordMute(note, { id: u.userId }, u.mutedWords).then(shouldMute => {
 				if (shouldMute) {
diff --git a/packages/backend/src/services/relay.ts b/packages/backend/src/services/relay.ts
index 725ce30ce..35779e79a 100644
--- a/packages/backend/src/services/relay.ts
+++ b/packages/backend/src/services/relay.ts
@@ -93,7 +93,7 @@ export async function relayRejected(id: string): Promise<string> {
 export async function deliverToRelays(user: { id: User['id']; host: null; }, activity: any): Promise<void> {
 	if (activity == null) return;
 
-	const relays = await relaysCache.fetch(null);
+	const relays = await relaysCache.fetch('');
 	if (relays == null || relays.length === 0) return;
 
 	// TODO
diff --git a/packages/backend/src/services/user-cache.ts b/packages/backend/src/services/user-cache.ts
index d67931c2c..2d2ec5711 100644
--- a/packages/backend/src/services/user-cache.ts
+++ b/packages/backend/src/services/user-cache.ts
@@ -6,15 +6,15 @@ import { subscriber } from '@/db/redis.js';
 
 export const userByIdCache = new Cache<CacheableUser>(
 	Infinity,
-	async (id) => id ? (await Users.findOneBy({ id }) ?? undefined) : undefined,
+	async (id) => await Users.findOneBy({ id }) ?? undefined,
 );
 export const localUserByNativeTokenCache = new Cache<CacheableLocalUser>(
 	Infinity,
-	async (token) => token ? (await Users.findOneBy({ token, host: IsNull() }) as ILocalUser | null ?? undefined) : undefined,
+	async (token) => await Users.findOneBy({ token, host: IsNull() }) as ILocalUser | null ?? undefined,
 );
 export const uriPersonCache = new Cache<CacheableUser>(
 	Infinity,
-	async (uri) => uri ? (await Users.findOneBy({ uri }) ?? undefined) : undefined,
+	async (uri) => await Users.findOneBy({ uri }) ?? undefined,
 );
 
 subscriber.on('message', async (_, data) => {