diff --git a/oidc_provider/migrations/0021_refresh_token_not_unique.py b/oidc_provider/migrations/0021_refresh_token_not_unique.py
new file mode 100644
index 0000000..691ae8e
--- /dev/null
+++ b/oidc_provider/migrations/0021_refresh_token_not_unique.py
@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10 on 2016-12-12 19:44
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('oidc_provider', '0020_client__post_logout_redirect_uris'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='token',
+            name='refresh_token',
+            field=models.CharField(default='', max_length=255, unique=True, verbose_name='Refresh Token'),
+            preserve_default=False,
+        ),
+    ]
diff --git a/oidc_provider/models.py b/oidc_provider/models.py
index ddc5016..9c456b5 100644
--- a/oidc_provider/models.py
+++ b/oidc_provider/models.py
@@ -125,7 +125,7 @@ class Code(BaseCodeTokenModel):
 class Token(BaseCodeTokenModel):
 
     access_token = models.CharField(max_length=255, unique=True, verbose_name=_(u'Access Token'))
-    refresh_token = models.CharField(max_length=255, unique=True, null=True, verbose_name=_(u'Refresh Token'))
+    refresh_token = models.CharField(max_length=255, unique=True, verbose_name=_(u'Refresh Token'))
     _id_token = models.TextField(verbose_name=_(u'ID Token'))
 
     def id_token():