A very common error among django learner is like this:
From django.db import models From django.contrib.auth.models import User

Create your models here.

class DLUser(models.Model):
xxxx = models.xxxxField()
user = models.OneToOneField('CustomUser', User)
...

When execute python manage.py validate you’ll get
Traceback (most recent call last): File "manage.py", line 13, in execute_manager(settings) File "C:\Python25\Lib\site-packages\django\core\management.py", line 1672, in execute_manager execute_from_command_line(action_mapping, argv) File "C:\Python25\Lib\site-packages\django\core\management.py", line 1569, in execute_from_command_line action_mapping[action]() File “C:\Python25\Lib\site-packages\django\core\management.py”, line 1152, in validate num_errors = get_validation_errors(outfile) File “C:\Python25\Lib\site-packages\django\core\management.py”, line 1018, in get_validation_errors for r in rel_opts.get_all_related_objects(): File “C:\Python25\Lib\site-packages\django\db\models\options.py”, line 125, in get_all_related_objects if f.rel and self == f.rel.to._meta: AttributeError: ’str’ object has no attribute ‘_meta’

The error message is misleading. Infact you just made a mistake here
user = models.OneToOneField('CustomUser', User)
Correct that to
user = models.OneToOneField(User, 'CustomUser')

Or, according to official defination of verbose-field-names, ForeignKey, ManyToManyField and OneToOneField require the first argument to be a model class, so use the verbose_name keyword argument:
user = models.OneToOneField(User, verbose_name='CustomUser')

And the problem is solved.