django1.9的用户认证auth 关于自定义扩展User报错，求解答

---报错内容start---
>>>python manage.py migrate
Operations to perform:
  Apply all migrations: admin, contenttypes, sql, auth, sessions
Running migrations:
  Rendering model states... DONE
  Applying admin.0001_initial...Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "C:\Python27\lib\site-packages\django\core\management\__init__.py", line 350, in execute_from_command_line
    utility.execute()
  File "C:\Python27\lib\site-packages\django\core\management\__init__.py", line 342, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "C:\Python27\lib\site-packages\django\core\management\base.py", line 348, in run_from_argv
    self.execute(*args, **cmd_options)
  File "C:\Python27\lib\site-packages\django\core\management\base.py", line 399, in execute
    output = self.handle(*args, **options)
  File "C:\Python27\lib\site-packages\django\core\management\commands\migrate.py", line 200, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "C:\Python27\lib\site-packages\django\db\migrations\executor.py", line 92, in migrate
    self._migrate_all_forwards(plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "C:\Python27\lib\site-packages\django\db\migrations\executor.py", line 121, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "C:\Python27\lib\site-packages\django\db\migrations\executor.py", line 198, in apply_migration
    state = migration.apply(state, schema_editor)
  File "C:\Python27\lib\site-packages\django\db\migrations\migration.py", line 123, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "C:\Python27\lib\site-packages\django\db\migrations\operations\models.py", line 59, in database_forwards
    schema_editor.create_model(model)
  File "C:\Python27\lib\site-packages\django\db\backends\base\schema.py", line 235, in create_model
    definition, extra_params = self.column_sql(model, field)
  File "C:\Python27\lib\site-packages\django\db\backends\base\schema.py", line 134, in column_sql
    db_params = field.db_parameters(connection=self.connection)
  File "C:\Python27\lib\site-packages\django\db\models\fields\related.py", line 962, in db_parameters
    return {"type": self.db_type(connection), "check": []}
  File "C:\Python27\lib\site-packages\django\db\models\fields\related.py", line 953, in db_type
    rel_field = self.target_field
  File "C:\Python27\lib\site-packages\django\db\models\fields\related.py", line 856, in target_field
    return self.foreign_related_fields[0]
  File "C:\Python27\lib\site-packages\django\db\models\fields\related.py", line 591, in foreign_related_fields
    return tuple(rhs_field for lhs_field, rhs_field in self.related_fields if rhs_field)
  File "C:\Python27\lib\site-packages\django\db\models\fields\related.py", line 578, in related_fields
    self._related_fields = self.resolve_related_fields()
  File "C:\Python27\lib\site-packages\django\db\models\fields\related.py", line 563, in resolve_related_fields
    raise ValueError('Related model %r cannot be resolved' % self.remote_field.model)
ValueError: Related model 'sql.User' cannot be resolved

#sql.models.py文件内容，sql是app的名字
from django.contrib.auth.models import AbstractUser

class User(AbstractUser):

    qq = models.CharField(max_length=16, blank=True, null=True, verbose_name=u'QQ')


#settings.py文件添加内容

AUTH_USER_MODEL = 'sql.User'