title: I faced to django.core.exceptions.ValidationError: ["'' value has an invalid format. It must be in YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format."]
body: <p>data model is below:</p>
  
  <pre><code>class User(models.Model):
      user_name = models.CharField(max_length=50)
      location = models.CharField(max_length=5)
      password = models.CharField(max_length=256,default='')
      last_time = models.DateTimeField(null=True,default='')
  
      def __str__(self):
          return self.user_name
  </code></pre>
  
  <p>I just tried python manage.py migrate.</p>
  
  <p>But, Django tells are...(I don't know what I make mistake.)</p>
  
  <pre><code>Operations to perform:
    Synchronize unmigrated apps: messages, staticfiles
    Apply all migrations: sessions, data, admin, books, auth, poll, contenttypes
  Synchronizing apps without migrations:
    Creating tables...
      Running deferred SQL...
    Installing custom SQL...
  Running migrations:
    Rendering model states... DONE
    Applying data.0003_user_last_time...Traceback (most recent call last):
    File "manage.py", line 10, in &lt;module&gt;
      execute_from_command_line(sys.argv)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/core/management/__init__.py", line 338, in execute_from_command_line
      utility.execute()
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/core/management/__init__.py", line 330, in execute
      self.fetch_command(subcommand).run_from_argv(self.argv)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/core/management/base.py", line 390, in run_from_argv
      self.execute(*args, **cmd_options)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/core/management/base.py", line 441, in execute
      output = self.handle(*args, **options)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/core/management/commands/migrate.py", line 221, in handle
      executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/migrations/executor.py", line 110, in migrate
      self.apply_migration(states[migration], migration, fake=fake, fake_initial=fake_initial)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/migrations/executor.py", line 147, in apply_migration
      state = migration.apply(state, schema_editor)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/migrations/migration.py", line 115, in apply
      operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/migrations/operations/fields.py", line 62, in database_forwards
      field,
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/backends/sqlite3/schema.py", line 179, in add_field
      self._remake_table(model, create_fields=[field])
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/backends/sqlite3/schema.py", line 77, in _remake_table
      self.effective_default(field)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 211, in effective_default
      default = field.get_db_prep_save(default, self.connection)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/models/fields/__init__.py", line 710, in get_db_prep_save
      prepared=False)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/models/fields/__init__.py", line 1482, in get_db_prep_value
      value = self.get_prep_value(value)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/models/fields/__init__.py", line 1461, in get_prep_value
      value = super(DateTimeField, self).get_prep_value(value)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/models/fields/__init__.py", line 1317, in get_prep_value
      return self.to_python(value)
    File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/django/db/models/fields/__init__.py", line 1444, in to_python
      params={'value': value},
  django.core.exceptions.ValidationError: ["'' value has an invalid format. It must be in YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format."]
  </code></pre>