diff --git a/django/contrib/auth/management/commands/createsuperuser.py b/django/contrib/auth/management/commands/createsuperuser.py
index d8023428e1..c5f6469548 100644
--- a/django/contrib/auth/management/commands/createsuperuser.py
+++ b/django/contrib/auth/management/commands/createsuperuser.py
@@ -62,7 +62,7 @@ class Command(BaseCommand):
                         other_data[field_name] = field.clean(options[field_name], None)
                     else:
                         raise CommandError("You must use --%s with --noinput." % field_name)
-            except exceptions.ValidationError, e:
+            except exceptions.ValidationError as e:
                 raise CommandError('; '.join(e.messages))
 
         else:
@@ -84,7 +84,7 @@ class Command(BaseCommand):
                         username = default_username
                     try:
                         username = username_field.clean(raw_value, None)
-                    except exceptions.ValidationError, e:
+                    except exceptions.ValidationError as e:
                         self.stderr.write("Error: %s" % '; '.join(e.messages))
                         username = None
                         continue
@@ -105,7 +105,7 @@ class Command(BaseCommand):
                         raw_value = input(capfirst(field.verbose_name + ': '))
                         try:
                             other_data[field_name] = field.clean(raw_value, None)
-                        except exceptions.ValidationError, e:
+                        except exceptions.ValidationError as e:
                             self.stderr.write("Error: %s" % '; '.join(e.messages))
                             other_data[field_name] = None
 
diff --git a/django/contrib/auth/models.py b/django/contrib/auth/models.py
index 076e7a9910..7f5f5f6801 100644
--- a/django/contrib/auth/models.py
+++ b/django/contrib/auth/models.py
@@ -289,7 +289,7 @@ class AbstractUser(AbstractBaseUser):
         help_text=_('Required. 30 characters or fewer. Letters, numbers and '
                     '@/./+/-/_ characters'),
         validators=[
-            validators.RegexValidator(re.compile('^[\w.@+-]+$'), _(u'Enter a valid username.'), 'invalid')
+            validators.RegexValidator(re.compile('^[\w.@+-]+$'), _('Enter a valid username.'), 'invalid')
         ])
     first_name = models.CharField(_('first name'), max_length=30, blank=True)
     last_name = models.CharField(_('last name'), max_length=30, blank=True)
diff --git a/django/db/models/base.py b/django/db/models/base.py
index b77d2d0760..a1f9e2f26e 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -110,7 +110,7 @@ class ModelBase(type):
 
         # If the model is a proxy, ensure that the base class
         # hasn't been swapped out.
-        if is_proxy and base_meta.swapped:
+        if is_proxy and base_meta and base_meta.swapped:
             raise TypeError("%s cannot proxy the swapped model '%s'." % (name, base_meta.swapped))
 
         if getattr(new_class, '_default_manager', None):