Django model with JSONField: Why do I get a CommandError when trying to dump data in XML?

Question

I have a Django model with a JSONField to handle multilingual text as follows:

from django.contrib.postgres.fields import JSONField

def default_language_JSON():
        content = {}
        for lang in settings.LANGUAGES:
                content[lang[0]] = ''
        return content

class Entity(PolymorphicModel, ShowFieldType):
        displayedNames = JSONField(
                null = True,
                blank = True,
                verbose_name=_('Displayed names'),
                help_text= _('These names are usually created automatically'),
                default = default_language_JSON
        )
...

When I try to dump the data with:

python3 manage.py dumpdata --natural-foreign --indent 4 --format=xml --verbosity 1 -o Database.xml

I get the error:

CommandError: Unable to serialize database: expected string or bytes-like object

I suppose it may have to do with the way the JSONField is serialized and I suspect the answer might be the encoder that should be used (as described in the documentation)

If I try dumping data in JSON or YAML no such error appears.

I use Django 2.1.4 in Ubuntu 16.04 with PostgreSQL 9.5

Any ideas?