Django-MarkItUp: Exception while starting tests (AttributeError: 'str' object has no attribute 'raw')

I can't find the solution for an exception when I start my tests which suddenly accured while coding. The website still works fine with runserver. Here is the full traceback:

Traceback (most recent call last):
  File "manage.py", line 10, in <module    
    execute_from_command_line(sys.argv)
  File "…/lib/python3.5/site-packages/django/core/management/__init__.py", line 367, in execute_from_command_line
    utility.execute()
  File "…/lib/python3.5/site-packages/django/core/management/__init__.py", line 359, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "…/lib/python3.5/site-packages/django/core/management/commands/test.py", line 29, in run_from_argv
    super(Command, self).run_from_argv(argv)
  File "…/lib/python3.5/site-packages/django/core/management/base.py", line 294, in run_from_argv
    self.execute(*args, **cmd_options)
  File "…/lib/python3.5/site-packages/django/core/management/base.py", line 345, in execute
    output = self.handle(*args, **options)
  File "…/lib/python3.5/site-packages/django/core/management/commands/test.py", line 72, in handle
    failures = test_runner.run_tests(test_labels)
  File "…/lib/python3.5/site-packages/django/test/runner.py", line 549, in run_tests
    old_config = self.setup_databases()
  File "…/lib/python3.5/site-packages/django/test/runner.py", line 499, in setup_databases
    self.parallel, **kwargs
  File "…/lib/python3.5/site-packages/django/test/runner.py", line 743, in setup_databases
    serialize=connection.settings_dict.get("TEST", {}).get("SERIALIZE", True),
  File "…/lib/python3.5/site-packages/django/db/backends/base/creation.py", line 78, in create_test_db
    self.connection._test_serialized_contents = self.serialize_db_to_string()
  File "…/lib/python3.5/site-packages/django/db/backends/base/creation.py", line 122, in serialize_db_to_string
    serializers.serialize("json", get_objects(), indent=None, stream=out)
  File "…/lib/python3.5/site-packages/django/core/serializers/__init__.py", line 129, in serialize
    s.serialize(queryset, **options)
  File "…/lib/python3.5/site-packages/django/core/serializers/base.py", line 88, in serialize
    self.handle_field(obj, field)
  File "…/lib/python3.5/site-packages/django/core/serializers/python.py", line 55, in handle_field
    self._current[field.name] = field.value_to_string(obj)
  File "…/lib/python3.5/site-packages/markitup/fields.py", line 103, in value_to_string
    return value.raw
AttributeError: 'str' object has no attribute 'raw'

Normally I try to narrow the issue down myself. But in this case I don't get anywhere.

I didn't update any packages. These are the packages in question:

• Django (1.10.7)
• django-markitup (3.0.0)

(The exception disappears when I replace my MarkupField with a standard TextField in models.py.)

Update: The error appears when I use --keepdb successively.

Seems you can fix it by

class MyMarkupField(MarkupField):
    def value_to_string(self, obj):
        value = self._get_val_from_obj(obj)
        if hasattr(value, "raw"):
            return value.raw
        return value

Then

field = MyMarkupField()

Actually i dont know why this error is coming.