diff --git a/src/misc/convert-host.ts b/src/misc/convert-host.ts
index dbf786455..a5fb15c66 100644
--- a/src/misc/convert-host.ts
+++ b/src/misc/convert-host.ts
@@ -19,3 +19,8 @@ export function extractDbHost(uri: string) {
 export function toPuny(host: string) {
 	return toASCII(host.toLowerCase());
 }
+
+export function toPunyNullable(host: string | null | undefined): string | null {
+	if (host == null) return null;
+	return toASCII(host.toLowerCase());
+}
diff --git a/src/queue/processors/inbox.ts b/src/queue/processors/inbox.ts
index 4deaef2ae..05fed0566 100644
--- a/src/queue/processors/inbox.ts
+++ b/src/queue/processors/inbox.ts
@@ -12,7 +12,7 @@ import { Instances, Users, UserPublickeys } from '../../models';
 import { instanceChart } from '../../services/chart';
 import { UserPublickey } from '../../models/entities/user-publickey';
 import fetchMeta from '../../misc/fetch-meta';
-import { toPuny } from '../../misc/convert-host';
+import { toPuny, toPunyNullable } from '../../misc/convert-host';
 import { validActor } from '../../remote/activitypub/type';
 import { ensure } from '../../prelude/ensure';
 
@@ -36,7 +36,7 @@ export default async (job: Bull.Job): Promise<void> => {
 
 	if (keyIdLower.startsWith('acct:')) {
 		const acct = parseAcct(keyIdLower.slice('acct:'.length));
-		const host = acct.host ? toPuny(acct.host) : null;
+		const host = toPunyNullable(acct.host);
 		const username = toPuny(acct.username);
 
 		if (host === null) {
diff --git a/src/server/api/endpoints/admin/emoji/list.ts b/src/server/api/endpoints/admin/emoji/list.ts
index cf73e4cc7..54686a5c5 100644
--- a/src/server/api/endpoints/admin/emoji/list.ts
+++ b/src/server/api/endpoints/admin/emoji/list.ts
@@ -1,7 +1,7 @@
 import $ from 'cafy';
 import define from '../../../define';
 import { Emojis } from '../../../../../models';
-import { toPuny } from '../../../../../misc/convert-host';
+import { toPunyNullable } from '../../../../../misc/convert-host';
 
 export const meta = {
 	desc: {
@@ -23,7 +23,7 @@ export const meta = {
 
 export default define(meta, async (ps) => {
 	const emojis = await Emojis.find({
-		host: ps.host ? toPuny(ps.host) : null
+		host: toPunyNullable(ps.host)
 	});
 
 	return emojis.map(e => ({
diff --git a/src/server/api/endpoints/users/followers.ts b/src/server/api/endpoints/users/followers.ts
index 88a474be7..0cb68353c 100644
--- a/src/server/api/endpoints/users/followers.ts
+++ b/src/server/api/endpoints/users/followers.ts
@@ -4,7 +4,7 @@ import define from '../../define';
 import { ApiError } from '../../error';
 import { Users, Followings } from '../../../../models';
 import { makePaginationQuery } from '../../common/make-pagination-query';
-import { toPuny } from '../../../../misc/convert-host';
+import { toPunyNullable } from '../../../../misc/convert-host';
 
 export const meta = {
 	desc: {
@@ -66,7 +66,7 @@ export const meta = {
 export default define(meta, async (ps, me) => {
 	const user = await Users.findOne(ps.userId != null
 		? { id: ps.userId }
-		: { usernameLower: ps.username!.toLowerCase(), host: toPuny(ps.host!) });
+		: { usernameLower: ps.username!.toLowerCase(), host: toPunyNullable(ps.host) });
 
 	if (user == null) {
 		throw new ApiError(meta.errors.noSuchUser);
diff --git a/src/server/api/endpoints/users/following.ts b/src/server/api/endpoints/users/following.ts
index 5e017150e..2e273dc0c 100644
--- a/src/server/api/endpoints/users/following.ts
+++ b/src/server/api/endpoints/users/following.ts
@@ -4,7 +4,7 @@ import define from '../../define';
 import { ApiError } from '../../error';
 import { Users, Followings } from '../../../../models';
 import { makePaginationQuery } from '../../common/make-pagination-query';
-import { toPuny } from '../../../../misc/convert-host';
+import { toPunyNullable } from '../../../../misc/convert-host';
 
 export const meta = {
 	desc: {
@@ -66,7 +66,7 @@ export const meta = {
 export default define(meta, async (ps, me) => {
 	const user = await Users.findOne(ps.userId != null
 		? { id: ps.userId }
-		: { usernameLower: ps.username!.toLowerCase(), host: toPuny(ps.host!) });
+		: { usernameLower: ps.username!.toLowerCase(), host: toPunyNullable(ps.host) });
 
 	if (user == null) {
 		throw new ApiError(meta.errors.noSuchUser);
diff --git a/src/server/api/private/signup.ts b/src/server/api/private/signup.ts
index 80f2d87b1..f8dba2eb2 100644
--- a/src/server/api/private/signup.ts
+++ b/src/server/api/private/signup.ts
@@ -10,7 +10,7 @@ import { genId } from '../../../misc/gen-id';
 import { usersChart } from '../../../services/chart';
 import { User } from '../../../models/entities/user';
 import { UserKeypair } from '../../../models/entities/user-keypair';
-import { toPuny } from '../../../misc/convert-host';
+import { toPunyNullable } from '../../../misc/convert-host';
 import { UserProfile } from '../../../models/entities/user-profile';
 import { getConnection } from 'typeorm';
 
@@ -109,7 +109,7 @@ export default async (ctx: Koa.BaseContext) => {
 			createdAt: new Date(),
 			username: username,
 			usernameLower: username.toLowerCase(),
-			host: host ? toPuny(host) : null,
+			host: toPunyNullable(host),
 			token: secret,
 			isAdmin: config.autoAdmin && usersCount === 0,
 		}));