diff --git a/problem/migrations/0004_auto_20150812_2254.py b/problem/migrations/0004_auto_20150812_2254.py
new file mode 100644
index 0000000..75f85d4
--- /dev/null
+++ b/problem/migrations/0004_auto_20150812_2254.py
@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('problem', '0003_auto_20150810_2233'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='problem',
+            name='tags',
+            field=models.ManyToManyField(to='problem.ProblemTag'),
+        ),
+    ]
diff --git a/problem/models.py b/problem/models.py
index b9a3181..aaa5ce0 100644
--- a/problem/models.py
+++ b/problem/models.py
@@ -49,4 +49,4 @@ class Problem(AbstractProblem):
     # 难度 0 - n
     difficulty = models.IntegerField()
     # 标签
-    tags = models.ManyToManyField(ProblemTag, null=True)
+    tags = models.ManyToManyField(ProblemTag)