Item.tax_rate should not be nullable

src/pretix/base/migrations/0007_auto_20160211_1710.py

@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9 on 2016-02-11 17:10
+from __future__ import unicode_literals
+
+from decimal import Decimal
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0006_auto_20160211_1630'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='item',
+            name='tax_rate',
+            field=models.DecimalField(decimal_places=2, default=Decimal('0.00'), max_digits=7, verbose_name='Taxes included in percent'),
+        ),
+    ]

src/pretix/base/models/items.py

@@ -1,5 +1,6 @@
 import sys
 from datetime import datetime
+from decimal import Decimal
 
 from django.db import models
 from django.db.models import Q, Case, Count, Sum, When
@@ -132,9 +133,9 @@ class Item(LoggedModel):
         max_digits=7, decimal_places=2, null=True
     )
     tax_rate = models.DecimalField(
-        null=True, blank=True,
         verbose_name=_("Taxes included in percent"),
-        max_digits=7, decimal_places=2
+        max_digits=7, decimal_places=2,
+        default=Decimal('0.00')
     )
     admission = models.BooleanField(
         verbose_name=_("Is an admission ticket"),