title: django.db.utils.IntegrityError: column "color_set_id" contains null values
body: <p>app.models.py</p>
  
  <pre><code>class ColorSet(models.Model):
      color = models.CharField(max_length=50, verbose_name='Color', default='', blank=True, null=True)
      code = models.CharField(max_length=50, verbose_name='Code of color', default='', blank=True, null=True)
  
      class Meta:
          verbose_name = 'Color'
          verbose_name_plural = 'Colors'
  </code></pre>
  
  <p>Also project.models.py</p>
  
  <pre><code>class Product(models.Model):
      title = models.CharField(max_length=200, verbose_name='Title of product')
      slug_field = models.SlugField(max_length=50, verbose_name='Slug', default='')
      description = models.TextField(verbose_name='Description')
      color_set = models.ForeignKey(ColorSet, verbose_name='Color', default='', blank=True, null=True)
  
      def __str__(self):
          return '%s %s %s' % (self.title, '-&gt;', self.category)
  
      class Meta:
          verbose_name = "Product"
          verbose_name_plural = "Products"
  </code></pre>
  
  <p>If I am doing 'migrate', I see something like this</p>
  
  <pre><code>Traceback (most recent call last):
  File "manage.py", line 22, in &lt;module&gt;
    execute_from_command_line(sys.argv)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/core/management/__init__.py", line 363, in execute_from_command_line
    utility.execute()
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/core/management/__init__.py", line 355, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/core/management/commands/migrate.py", line 204, in handle
    fake_initial=fake_initial,
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/db/migrations/executor.py", line 115, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/db/migrations/executor.py", line 145, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/db/migrations/migration.py", line 129, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/db/migrations/operations/fields.py", line 87, in database_forwards
    field,
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 429, in add_field
    self.execute(sql, params)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 120, in execute
    cursor.execute(sql, params)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/db/backends/utils.py", line 80, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "/home/vladislav/.local/lib/python3.5/site-packages/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  django.db.utils.IntegrityError: column "color_set_id" contains null values
  </code></pre>
  
  <p>I tried to add 'blank = True, null = True', but it's didn't help me.
  How can I fix this problem?</p>