diff --git a/openwisp_controller/config/migrations/0013_auto_20180918_1055.py b/openwisp_controller/config/migrations/0013_auto_20180918_1055.py
new file mode 100644
index 000000000..ada7d0fb0
--- /dev/null
+++ b/openwisp_controller/config/migrations/0013_auto_20180918_1055.py
@@ -0,0 +1,18 @@
+# Generated by Django 2.0.8 on 2018-09-18 08:55
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('config', '0012_auto_20180219_1501'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='device',
+            name='notes',
+            field=models.TextField(blank=True, help_text='internal notes'),
+        ),
+    ]
diff --git a/requirements.txt b/requirements.txt
index 7b9055a9c..09b1adbfa 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,3 +2,4 @@ django-netjsonconfig>=0.8.0,<0.9.0
 openwisp-utils[users]<0.3
 django-loci>=0.1.1,<0.3.0
 djangorestframework-gis>=0.12.0,<0.13.0
+six>=1.11.0