diff --git a/archivebox/core/migrations/0004_auto_20200713_1552.py b/archivebox/core/migrations/0004_auto_20200713_1552.py
new file mode 100644
index 00000000..69836623
--- /dev/null
+++ b/archivebox/core/migrations/0004_auto_20200713_1552.py
@@ -0,0 +1,19 @@
+# Generated by Django 3.0.7 on 2020-07-13 15:52
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0003_auto_20200630_1034'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='snapshot',
+            name='timestamp',
+            field=models.CharField(db_index=True, default=None, max_length=32, unique=True),
+            preserve_default=False,
+        ),
+    ]
diff --git a/archivebox/core/models.py b/archivebox/core/models.py
index 42929e5a..7ac9427b 100644
--- a/archivebox/core/models.py
+++ b/archivebox/core/models.py
@@ -13,7 +13,7 @@ class Snapshot(models.Model):
     id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
 
     url = models.URLField(unique=True)
-    timestamp = models.CharField(max_length=32, null=True, default=None, db_index=True)
+    timestamp = models.CharField(max_length=32, unique=True, db_index=True)
 
     title = models.CharField(max_length=128, null=True, default=None, db_index=True)
     tags = models.CharField(max_length=256, null=True, default=None, db_index=True)