diff --git a/src/api/endpoints/app/show.ts b/src/api/endpoints/app/show.ts
index cfb03bb9e5..6b6e4f010c 100644
--- a/src/api/endpoints/app/show.ts
+++ b/src/api/endpoints/app/show.ts
@@ -57,12 +57,12 @@ module.exports = (params, user, _, isSecure) =>
 	const [nameId, nameIdErr] = it(params.name_id, 'string');
 	if (nameIdErr) return rej('invalid name_id param');
 
-	if (appId === null && nameId === null) {
+	if (appId === undefined && nameId === undefined) {
 		return rej('app_id or name_id is required');
 	}
 
 	// Lookup app
-	const app = appId !== null
+	const app = appId !== undefined
 		? await App.findOne({ _id: appId })
 		: await App.findOne({ name_id_lower: nameId.toLowerCase() });
 
diff --git a/src/api/endpoints/posts/create.ts b/src/api/endpoints/posts/create.ts
index 3dc121305c..d558cd3018 100644
--- a/src/api/endpoints/posts/create.ts
+++ b/src/api/endpoints/posts/create.ts
@@ -35,7 +35,7 @@ module.exports = (params, user, app) =>
 	if (mediaIdsErr) return rej('invalid media_ids');
 
 	let files = [];
-	if (mediaIds !== null) {
+	if (mediaIds !== undefined) {
 		// Fetch files
 		// forEach だと途中でエラーなどがあっても return できないので
 		// 敢えて for を使っています。
@@ -67,7 +67,7 @@ module.exports = (params, user, app) =>
 	if (repostIdErr) return rej('invalid repost_id');
 
 	let repost = null;
-	if (repostId !== null) {
+	if (repostId !== undefined) {
 		// Fetch repost to post
 		repost = await Post.findOne({
 			_id: repostId
@@ -109,7 +109,7 @@ module.exports = (params, user, app) =>
 	if (inReplyToPostIdErr) return rej('invalid in_reply_to_post_id');
 
 	let inReplyToPost = null;
-	if (inReplyToPostId !== null) {
+	if (inReplyToPostId !== undefined) {
 		// Fetch reply
 		inReplyToPost = await Post.findOne({
 			_id: inReplyToPostId
@@ -130,7 +130,7 @@ module.exports = (params, user, app) =>
 	if (pollErr) return rej('invalid poll');
 
 	let poll = null;
-	if (_poll !== null) {
+	if (_poll !== undefined) {
 		const [pollChoices, pollChoicesErr] =
 			it(params.poll).expect.array()
 				.unique()
diff --git a/src/api/endpoints/users/posts.ts b/src/api/endpoints/users/posts.ts
index 770831c426..78f91b16c5 100644
--- a/src/api/endpoints/users/posts.ts
+++ b/src/api/endpoints/users/posts.ts
@@ -26,7 +26,7 @@ module.exports = (params, me) =>
 	const [username, usernameErr] = it(params.username, 'string');
 	if (usernameErr) return rej('invalid username param');
 
-	if (userId === null && username === null) {
+	if (userId === undefined && username === undefined) {
 		return rej('user_id or username is required');
 	}
 
@@ -55,7 +55,7 @@ module.exports = (params, me) =>
 		return rej('cannot set since_id and max_id');
 	}
 
-	const q = userId != null
+	const q = userId !== undefined
 		? { _id: userId }
 		: { username_lower: username.toLowerCase() } ;
 
diff --git a/src/api/endpoints/users/show.ts b/src/api/endpoints/users/show.ts
index cae4ac0b7f..9997d2ba03 100644
--- a/src/api/endpoints/users/show.ts
+++ b/src/api/endpoints/users/show.ts
@@ -25,13 +25,13 @@ module.exports = (params, me) =>
 	const [username, usernameErr] = it(params.username, 'string');
 	if (usernameErr) return rej('invalid username param');
 
-	if (userId === null && username === null) {
+	if (userId === undefined && username === undefined) {
 		return rej('user_id or username is required');
 	}
 
-	const q = userId != null
+	const q = userId !== undefined
 		? { _id: userId }
-		: { username_lower: username.toLowerCase() } ;
+		: { username_lower: username.toLowerCase() };
 
 	// Lookup user
 	const user = await User.findOne(q, {