diff --git a/problem/migrations/0017_alter_problem_last_update_time.py b/problem/migrations/0017_alter_problem_last_update_time.py
new file mode 100644
index 0000000..63e2a35
--- /dev/null
+++ b/problem/migrations/0017_alter_problem_last_update_time.py
@@ -0,0 +1,18 @@
+# Generated by Django 5.0.6 on 2025-05-09 00:20
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('problem', '0016_alter_problem_last_update_time'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='problem',
+            name='last_update_time',
+            field=models.DateTimeField(auto_now=True, null=True),
+        ),
+    ]
diff --git a/problem/models.py b/problem/models.py
index bd8fec7..1859825 100644
--- a/problem/models.py
+++ b/problem/models.py
@@ -60,7 +60,7 @@ class Problem(models.Model):
     template = JSONField()
     create_time = models.DateTimeField(auto_now_add=True)
     # we can not use auto_now here
-    last_update_time = models.DateTimeField(auto_now=True)
+    last_update_time = models.DateTimeField(auto_now=True, null=True)
     created_by = models.ForeignKey(User, on_delete=models.CASCADE)
     # ms
     time_limit = models.IntegerField()