diff --git a/src/server/web/docs.ts b/src/server/web/docs.ts
index 84dbe6f09..f823c308c 100644
--- a/src/server/web/docs.ts
+++ b/src/server/web/docs.ts
@@ -19,6 +19,14 @@ import endpoints from '../api/endpoints';
 const locales = require('../../../locales');
 import * as nestedProperty from 'nested-property';
 
+function getLang(lang: string): string {
+	if (['en-US', 'ja-JP'].includes(lang)) {
+		return lang;
+	} else {
+		return 'en-US';
+	}
+}
+
 async function genVars(lang: string): Promise<{ [key: string]: any }> {
 	const vars = {} as { [key: string]: any };
 
@@ -167,7 +175,7 @@ router.get('/assets/*', async ctx => {
 });
 
 router.get('/*/api/endpoints/*', async ctx => {
-	const lang = ctx.params[0];
+	const lang = getLang(ctx.params[0]);
 	const name = ctx.params[1];
 	const ep = endpoints.find(e => e.name === name);
 
@@ -194,7 +202,7 @@ router.get('/*/api/endpoints/*', async ctx => {
 });
 
 router.get('/*/api/entities/*', async ctx => {
-	const lang = ctx.params[0];
+	const lang = getLang(ctx.params[0]);
 	const entity = ctx.params[1];
 
 	const x = yaml.safeLoad(fs.readFileSync(path.resolve(`${__dirname}/../../../src/docs/api/entities/${entity}.yaml`), 'utf-8'));
@@ -211,7 +219,7 @@ router.get('/*/api/entities/*', async ctx => {
 });
 
 router.get('/*/*', async ctx => {
-	const lang = ctx.params[0];
+	const lang = getLang(ctx.params[0]);
 	const doc = ctx.params[1];
 
 	showdown.extension('urlExtension', () => ({