Django 1.9.8 Sitemaps is broken due to "isinstance() arg 2 must be a class, type, or tuple of classes and types"

[Chen Xu]

I am using Django version 1.9.8, and mezzanine version 4.1.0.
The following code failed:

from mezzanine.core.sitemaps import DisplayableSitemap
sitemaps = {"sitemaps": {"displayable": DisplayableSitemap, 'views': SwiftypeSitemap}}
urlpatterns += patterns("django.contrib.sitemaps.views", ("^sitemap\.xml$", "sitemap", sitemaps))

The trace back:

TypeError at /sitemap.xml
isinstance() arg 2 must be a class, type, or tuple of classes and types
Request Method:	GET
Request URL:	https://xxxxxxxxxx.com:8787/sitemap.xml
Django Version:	1.9.8
Exception Type:	TypeError
Exception Value:	
isinstance() arg 2 must be a class, type, or tuple of classes and types
Exception Location:	/srv/xxxxxxxxxx/virtualenvs/corpsite/local/lib/python2.7/site-packages/django/db/models/fields/related.py in get_default, line 908
Python Executable:	/srv/xxxxxxxxxx/virtualenvs/corpsite/bin/uwsgi
Python Version:	2.7.6
Python Path:	
['.',
 '',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/src/supervisor',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/lib/python2.7',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/lib/python2.7/plat-x86_64-linux-gnu',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/lib/python2.7/lib-tk',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/lib/python2.7/lib-old',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/lib/python2.7/lib-dynload',
 '/usr/lib/python2.7',
 '/usr/lib/python2.7/plat-x86_64-linux-gnu',
 '/usr/lib/python2.7/lib-tk',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/local/lib/python2.7/site-packages',
 '/srv/xxxxxxxxxx/www/private']
Server time:	Fri, 22 Jul 2016 17:48:22 -0700
...
...

/srv/xxxxxxxxxx/virtualenvs/corpsite/local/lib/python2.7/site-packages/django/db/models/fields/related.py in get_default
        if isinstance(field_default, self.remote_field.model): ...
▶ Local vars
Variable	Value
self: <django.db.models.fields.related.ForeignKey: site>
field_default: None

The problem is here: self.remote_field.model is not a type of class, but unicode.