diff --git a/packages/hydrooj/src/service/db.ts b/packages/hydrooj/src/service/db.ts
index 5bf541ae7..44bf07f3c 100644
--- a/packages/hydrooj/src/service/db.ts
+++ b/packages/hydrooj/src/service/db.ts
@@ -33,7 +33,8 @@ class MongoService implements BaseService {
         return mongourl;
     }
 
-    async start(opts: MongoConfig = {}) {
+    async start(opts: MongoConfig) {
+        opts ||= {};
         let mongourl = MongoService.buildUrl(opts);
         if (process.env.CI) {
             const { MongoMemoryServer } = require('mongodb-memory-server');