diff --git a/packages/hydrooj/package.json b/packages/hydrooj/package.json
index ff434890f..7291d349e 100644
--- a/packages/hydrooj/package.json
+++ b/packages/hydrooj/package.json
@@ -1,6 +1,6 @@
 {
     "name": "hydrooj",
-    "version": "4.2.2",
+    "version": "4.2.3",
     "bin": "bin/hydrooj.js",
     "main": "src/plugin-api",
     "module": "src/plugin-api",
diff --git a/packages/hydrooj/src/handler/problem.ts b/packages/hydrooj/src/handler/problem.ts
index dc7d7e367..473c4406b 100644
--- a/packages/hydrooj/src/handler/problem.ts
+++ b/packages/hydrooj/src/handler/problem.ts
@@ -484,6 +484,7 @@ export class ProblemSubmitHandler extends ProblemDetailHandler {
     @param('tid', Types.ObjectID, true)
     async prepare(domainId: string, tid?: ObjectID) {
         if (tid && !contest.isOngoing(this.tdoc, this.tsdoc)) throw new ContestNotLiveError(this.tdoc.docId);
+        if (typeof this.pdoc.config === 'string') throw new BadRequestError('Invalid problem config');
     }
 
     async get() {
diff --git a/packages/hydrooj/src/model/contest.ts b/packages/hydrooj/src/model/contest.ts
index f5da5ac81..63a779d50 100644
--- a/packages/hydrooj/src/model/contest.ts
+++ b/packages/hydrooj/src/model/contest.ts
@@ -130,9 +130,9 @@ const acm = buildContestRule({
         ];
         if (isExport) {
             row.push({ type: 'email', value: udoc.mail });
-            row.push({ type: 'string', value: udoc.school });
-            row.push({ type: 'string', value: udoc.displayName });
-            row.push({ type: 'string', value: udoc.studentId });
+            row.push({ type: 'string', value: udoc.school || '' });
+            row.push({ type: 'string', value: udoc.displayName || '' });
+            row.push({ type: 'string', value: udoc.studentId || '' });
         }
         row.push({
             type: 'time',
@@ -260,9 +260,9 @@ const oi = buildContestRule({
         ];
         if (isExport) {
             row.push({ type: 'email', value: udoc.mail });
-            row.push({ type: 'string', value: udoc.school });
-            row.push({ type: 'string', value: udoc.displayName });
-            row.push({ type: 'string', value: udoc.studentId });
+            row.push({ type: 'string', value: udoc.school || '' });
+            row.push({ type: 'string', value: udoc.displayName || '' });
+            row.push({ type: 'string', value: udoc.studentId || '' });
         }
         row.push({ type: 'total_score', value: tsdoc.score || 0 });
         for (const pid of tdoc.pids) {
diff --git a/packages/hydrooj/src/model/setting.ts b/packages/hydrooj/src/model/setting.ts
index 115c67490..6b3734716 100644
--- a/packages/hydrooj/src/model/setting.ts
+++ b/packages/hydrooj/src/model/setting.ts
@@ -164,7 +164,7 @@ DomainSetting(
 );
 
 DomainUserSetting(
-    Setting('setting_info', 'displayName', null, 'text', 'Display Name'),
+    Setting('setting_info', 'displayName', '', 'text', 'Display Name'),
     Setting('setting_storage', 'nAccept', 0, 'number', 'nAccept', null, FLAG_HIDDEN | FLAG_DISABLED),
     Setting('setting_storage', 'nSubmit', 0, 'number', 'nSubmit', null, FLAG_HIDDEN | FLAG_DISABLED),
     Setting('setting_storage', 'nLike', 0, 'number', 'nLike', null, FLAG_HIDDEN | FLAG_DISABLED),