diff --git a/framework/framework/validator.ts b/framework/framework/validator.ts
index 66510940d..07664834b 100644
--- a/framework/framework/validator.ts
+++ b/framework/framework/validator.ts
@@ -61,7 +61,7 @@ export interface Types {
     CommaSeperatedArray: Type<string[]>;
     Set: Type<Set<any>>;
     Any: Type<any>;
-    ArrayOf: <T extends Type<any>>(type: T) => (T extends Type<infer R> ? Type<R[]> : never);
+    ArrayOf: <T extends Type<any>>(type: T, isOptional?: boolean) => (T extends Type<infer R> ? Type<R[]> : never);
     AnyOf: <T extends Type<any>>(...type: T[]) => (T extends Type<infer R> ? Type<R> : never);
 }
 
@@ -175,15 +175,21 @@ export const Types = {
         (v) => emojiRegex().test(v.toString()),
     ],
     Any: [(v) => v, null],
-    ArrayOf: (type) => [
+    ArrayOf: (type, isOptional = false) => [
         (v) => {
             const arr = v instanceof Array ? v : [v];
-            return arr.map((i) => type[0](i));
+            return arr.map((i) => {
+                if (isOptional && [undefined, null, ''].includes(i)) return undefined;
+                return type[0](i);
+            });
         },
         (v) => {
             if (!type[1]) return true;
             const arr = v instanceof Array ? v : [v];
-            return arr.every((i) => type[1](i));
+            return arr.every((i) => {
+                if ([undefined, null, ''].includes(i)) return isOptional;
+                return type[1](i);
+            });
         },
     ] as any,
     AnyOf: (...types) => [
@@ -195,10 +201,4 @@ export const Types = {
 try {
     const { ObjectId } = require('mongodb');
     Types.ObjectId = [((v) => new ObjectId(v)) as any, ObjectId.isValid];
-} catch (e) {
-
-}
-
-// @ts-ignore
-Types.ObjectID = Types.ObjectId;
-// backward compatibility
+} catch (e) { }
diff --git a/packages/hydrooj/src/handler/problem.ts b/packages/hydrooj/src/handler/problem.ts
index dc3fa7a2a..f2fcfdbf9 100644
--- a/packages/hydrooj/src/handler/problem.ts
+++ b/packages/hydrooj/src/handler/problem.ts
@@ -483,7 +483,7 @@ export class ProblemSubmitHandler extends ProblemDetailHandler {
     @param('lang', Types.Name)
     @param('code', Types.String, true)
     @param('pretest', Types.Boolean)
-    @param('input', Types.ArrayOf(Types.String), true)
+    @param('input', Types.ArrayOf(Types.String, true), true)
     @param('tid', Types.ObjectId, true)
     async post(domainId: string, lang: string, code: string, pretest = false, input: string[] = [], tid?: ObjectId) {
         const config = this.pdoc.config;
@@ -499,6 +499,7 @@ export class ProblemSubmitHandler extends ProblemDetailHandler {
                 throw new ProblemNotAllowPretestError('type');
             }
             if (!input.length) throw new ValidationError('input');
+            input = input.map((i) => i || '');
         }
         await this.limitRate('add_record', 60, system.get('limit.submission_user'), '{{user}}');
         await this.limitRate('add_record', 60, pretest ? system.get('limit.pretest') : system.get('limit.submission'));