UnicodeError w/ Django, MySQL & custom unicode classes

keyvalue.py

# -*- coding: utf-8 -*-
from __future__ import absolute_import, unicode_literals
from django.db.models import TextField


class KeyValue(unicode):
    pass


class KeyValueField(TextField):
    
    def from_db_value(self, value, expression, connection, context):
        return KeyValue(value)
        

models.py

# -*- coding: utf-8 -*-
from __future__ import absolute_import, unicode_literals
from django.db import models
from keyvalue import KeyValueField


class MyFirstModel(models.Model):
    value1 = models.TextField(default='', blank=True)
    value2 = KeyValueField(default='', blank=True)
    

class MySecondModel(models.Model):
    value1 = models.TextField(default='', blank=True)
    
    

step1.py

# -*- coding: utf-8 -*-
from __future__ import absolute_import, unicode_literals
from .models import MyFirstModel


MyFirstModel.objects.create(
  value1="Hellö",
  value2="Hellö",
)

MySecondModel.objects.create(
  value="Test",
)

step2.py

# -*- coding: utf-8 -*-
from __future__ import absolute_import, unicode_literals
from .models import MyFirstModel, MySecondModel

first = MyFirstModel.objects.get(id=1)
second = MySecondModel.objects.get(id=1)

second.value = first.value1
second.save() # All fine!

second.value = first.value2
second.save() # UnicodeError