Integer field choice list keeps triggering migration

I have the following model and choice list:

BTW_TARIEF_CHOICES = {
    (1, '6%'),
    (2, '21%'),
    (3, '0%'),
}


class FactuurItem(models.Model):
    naam = models.CharField(max_length=100)
    factuur_naam = models.CharField(max_length=100)
    eenheid = models.CharField(max_length=10)
    prijs = models.DecimalField(max_digits=6, decimal_places=2)
    btw_tarief = models.IntegerField(choices=BTW_TARIEF_CHOICES, default=2)

    class Meta:
        ordering = ['naam']

    def __str__(self):
        return self.naam

Makemigrations creates a migration for the btw_tarief field every time I run it, even though the field hasn't changed. It shows up in every migration file that has been created since I added the field to the model. The only thing that seems different in every migration file is that the order of BTW_TARIEF_CHOICES is randomized:

class Migration(migrations.Migration):

    dependencies = [
        ('common', '0021_auto_20160503_0849'),
    ]

    operations = [
        migrations.AlterField(
            model_name='factuuritem',
            name='btw_tarief',
            field=models.IntegerField(choices=[(1, '6%'), (3, '0%'), (2, '21%')], default=2),
        ),
    ]

class Migration(migrations.Migration):

    dependencies = [
        ('common', '0020_auto_20160502_1656'),
    ]

    operations = [
        migrations.AlterField(
            model_name='factuuritem',
            name='btw_tarief',
            field=models.IntegerField(choices=[(2, '21%'), (3, '0%'), (1, '6%')], default=2),
        ),
    ]

class Migration(migrations.Migration):

    dependencies = [
        ('common', '0019_auto_20160502_1604'),
    ]

    operations = [
        migrations.AlterField(
            model_name='factuuritem',
            name='btw_tarief',
            field=models.IntegerField(choices=[(1, '6%'), (3, '0%'), (2, '21%')], default=2),
        ),
    ]

And so on... This happens on Django 1.9.5 and 1.9.6 .

Any ideas why this might be happening?

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