AttributeError: 'user' object has no attribute 'is_admin'

I customized the user model by extending AbstractBaseUser which accepts only as email id. Here is the model:

class User(AbstractBaseUser, PermissionsMixin):
    email = models.EmailField('email address', unique=True)
    first_name = models.CharField('first name', max_length=30, blank=True)
    last_name = models.CharField('last name', max_length=30, blank=True)
    date_joined = models.DateTimeField('date joined', auto_now_add=True)
    is_active = models.BooleanField('active', default=True)
    is_staff = models.BooleanField(default=False)

    objects = UserManager()

    USERNAME_FIELD = 'email'
    REQUIRED_FIELDS = []

class Meta:
    verbose_name = 'user'
    verbose_name_plural = 'users'

def get_full_name(self):
    full_name = '%s %s' % (self.first_name, self.last_name)
    return full_name.strip()

def get_short_name(self):
    return self.first_name

def email_user(self, subject, message, from_email=None, **kwargs):
    send_mail(subject, message, from_email, [self.email], **kwargs)

The model manager for the above model is:

class UserManager(BaseUserManager):
use_in_migrations = True

def _create_user(self, email, password, **extra_fields):
    """
    Creates and saves a User with the given email and password.
    """
    if not email:
        raise ValueError('The given email must be set')
    email = self.normalize_email(email)
    user = self.model(email=email, **extra_fields)
    user.set_password(password)
    user.save(using=self._db)
    return user

def create_user(self, email, password=None, **extra_fields):
    extra_fields.setdefault('is_superuser', False)
    return self._create_user(email, password, **extra_fields)

def create_superuser(self, email, password, **extra_fields):
    extra_fields.setdefault('is_superuser', True)

    if extra_fields.get('is_superuser') is not True:
        raise ValueError('Superuser must have is_superuser=True.')

    return self._create_user(email, password, **extra_fields)

I can create a custom user, but I get an attribute error when trying to log in:

Attribute error: 'User' object has no attribute 'is_admin'

Below is the admin.py file

class ProfileInline(admin.StackedInline):
model = Course
can_delete = False
verbose_name_plural = 'Course'
fk_name = 'user'

class CustomUserAdmin(UserAdmin):
    inlines = (ProfileInline, )

    def get_inline_instances(self, request, obj=None):
        if not obj:
            return list()
    return super(CustomUserAdmin, self).get_inline_instances(request, obj)


admin.site.register(User, CustomUserAdmin)