python - TypeError: coercing to Unicode: need string or buffer, NoneType found -

i'm wading throught django tutorial, part 2: https://docs.djangoproject.com/en/1.9/intro/tutorial02/ , in place attach several choices 1 question

# display choices related object set -- none far. >>> q.choice_set.all() [] 

i got error:

/home/paulmad/env/local/lib/python2.7/site-packages/ipython/lib/pretty.pyc in _repr_pprint(obj, p, cycle)     692     """a pprint redirects normal repr function."""     693     # find newlines , replace them p.break_() --> 694     output = repr(obj)     695     idx,output_line in enumerate(output.splitlines()):     696         if idx:  /home/paulmad/env/local/lib/python2.7/site-packages/django/db/models/query.pyc in __repr__(self)     235         if len(data) > repr_output_size:     236             data[-1] = "...(remaining elements truncated)..." --> 237         return repr(data)     238      239     def __len__(self):  /home/paulmad/env/local/lib/python2.7/site-packages/django/db/models/base.pyc in __repr__(self)     457     def __repr__(self):     458         try: --> 459             u = six.text_type(self)     460         except (unicodeencodeerror, unicodedecodeerror):     461             u = '[bad unicode data]'  typeerror: coercing unicode: need string or buffer, nonetype found 

here models:

from __future__ import unicode_literals  import datetime django.db import models django.utils import timezone django.utils.encoding import python_2_unicode_compatible # create models here.  @python_2_unicode_compatible class question(models.model):     question_text = models.charfield(max_length=200)     pub_date = models.datetimefield('date published')      def __str__(self):         return self.question_text      def __unicode__(self):         return unicode(self.quesion_text) or u''      def was_published_recently(self):         return self.pub_date >= timezone.now() - datetime.timedelta(days=1)  @python_2_unicode_compatible class choice(models.model):     question = models.foreignkey(question, on_delete=models.cascade)     choice_text = models.charfield(max_length=200)     votes = models.integerfield(default=0)      def __str__(self):         self.choice_text      def __unicode__(self):         return unicode(self.choice_text) or u'' 

django 1.9.1, how resolve problem?

in model, update method called __str__(self).

class choice(models.model):     choice_text = models.charfield(max_length=255, null=true)      def __str__(self):        return self.choice_text if self.choice_next else ''